Consolidate thread show/hide logic into ThreadController

This template conditional is notorious for accumulating extra terms, so
this commit rolls it into ThreadController where it can be more easily
tested.

T89 combined scope save btn

When changing permissions via the dropdown menu,
the changed permissions were applied to the _domain model_
representing the saved annotation instead of the _view model_
representing the current annotation being edited.

The change to the domain model triggered a reset of the view
model via AnnotationController.render(), losing any unsaved edits.

This commit makes AnnotationController.setPrivacy() apply permissions
changes to the view model in the same way as other edits in the
annotation view. These then only take effect when the 'Post to'
button is clicked.

Fixes #2567

Instead of patching the upstream UI Bootstrap build, add a wrapper around
it which stubs the $templateRequest service and wraps the Angular 1.2x implementation
of $animate.(add|remove)Class with one which returns a Promise as
UI Bootstrap expects.

When the app is upgraded to Angular 1.3x+, the 'ui-bootstrap-custom' module can just
be removed.

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When transforming old-style comments, handle null targets

Make activation created_by/valid_until fields nullable

Fix a bug where canceling changes did not reset the text

AnnotationController.revert() reverted changes to the
annotation in the editor by copying across properties
from the saved annotation to the draft.

Any properties which were _added_ in the draft but not
set in the saved version, such as the text when the text
is empty, were not reverted.

Fixes #2561

Add the updated group icon and update the icomoon font.
Also simplify the instructions for updating the icomoon font.

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In Chrome, clicks on elements inside a disabled <button>
will not invoke the <button>'s "click" event listeners
but they will propagate through the button and invoke parent
element's click handlers. In Firefox however, the click
event is not propagated.

The fix here is to install the click handler for the main
part of a dropdown button on the <button> itself, rather than
the container <div> which holds the main button and
the dropdown arrow.

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The spacing was lost when the cancel button was moved
into the <publish-annotation-btn> directive.

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Fix email notifications

Add guidelines for writing model code and db migrations

Update instructions for running front-end tests.

Make auth.effective_principals do what it says

 * Use cancel button with a thin circle outline

 * Change normal and hover colors to light/dark grey as
   per discussion on the card.

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Automatically convert URLs in annotation text to links

Add a cancel button with a thin circle outline which
matches the design.

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This text has been obsoleted by the new 'Post to XXX' design
which makes it clearer who new annotations will be visible to.

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Only URLs with a scheme or beginning with 'www.' are processed.

Fixes #2505

Instead of passing a full object of attributes to
util.createDirective() at the start of each test, render
a default instance in beforeEach() and then just relink
the element with the changes for a specific test.

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When false, both the dropdown menu and the post button are disabled.

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The <privacy> drop-down has been replaced by the
new combined Post / privacy button.

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Re-implement the behavior where new annotations
use the last-used privacy setting by default.

This was previously implemented in privacy.js
and the logic now lives in the replacement component,
publish-annotation-btn.js. However, the defaults
really shouldn't be set in the view but earlier when
the new annotation is created.

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The newer version of Angular UI bootstrap has
an optional keyboard-nav directive to
make dropdowns keyboard accessible.

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Using a <button> gives us keyboard accessibility and focus.

The downside is that additional styling is needed to suppress
default styling from the UA.

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This is a more descriptive name explaining what the component
is for.

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 * Move the annotation privacy label below the button. The label's
   height may vary depending on the selected option, so moving it
   below the button avoids changing the button's position when
   a different option is selected from the dropdown.

 * Remove the previous separate privacy dropdown and replace with
   the combined button.

 * Use flexbox to simplify layout of the annotation form actions

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This directive provides a button to publish an annotation
plus a dropdown-menu for changing its privacy settings.

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h/static/scripts/vendor/angular-bootstrap.js contains
a legacy version of the dropdown component

ui-bootstrap contains the current version with additional features
including the ability to open and close the dropdown via an
'is-open' attribute, keyboard navigation support and the ability
to use it via attribute directives.

This build has been lightly edited for Angular 1.2.x compatibility
(the original build requires a couple of features from Angular 1.3)

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Due to the existence of a 'p + p' selector in app.css, the <p>
element containing the 'This annotation is visible only to you'
label was displayed at a different vertical position to
the alternative labels.

Use ng-if rather than ng-show to remove the non-visible
labels so that the rendered <p> is always the first <p> child
of its parent and therefore always rendered at the same position.

The annotation action links were too small to be easily
legible, as was the 'This annotation will be visible to ...' label.

This sets the size of 'small' text to $body1-font-size which is
the smallest legible size for text in the app.

<primary-action-btn> is the purely visual component which
provides a button with a dropdown arrow menu that will open
an associated dropdown menu.

An additional component or controller will then wrap this
and provide the actual Save functionality.

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