Re-implement hiliting annotation cards on hover

Remove strings from the privacy template

Instead we use a method on the scope to determine the current
permission level.

Focus share link when pop-up opens

Sticky privacy on the browser

Use an object for privacy level models

Provide a feedback button in the interface

This makes the association between the level key name and the display
text explicit and tight in the code.

Meant to resolve #1653. As per @dwhly's suggestion the dropdown menu is now accessible even when not logged in. Note: items relevant to only logged in users are not shown.

So that
 - Instead of a map, we now store a list of focused tags
 - The check for being focused has been moved
   into a method in the controller
 - The sidebar now exepts simply a list of tags to
   focus on (no more add/remove tags to existing list)

Don't bother looking up the annotations (for the tags),
since we only need the list of tags anyway.

(It was different when we worked with the IDs, but we
moved to use tags instead, so the actual annotations
are no longer needed.)

Restructure Chrome Extension

... because new and deleted annotations don't have IDs,
only tags, assigned by the bridge.

- The permission plugin no longer sets the permissions
- Read permission is a one-item list ["group:__world__"] instead of ["group:__world__", <username>"]
- Privacy directive moved to directives/privacy.coffee
- Uses the simple localStorage solution inside the privacy directive.

... when hovering over the highlights in the document,
or over the annotation cards themselves.

Consistent names for focusing on annotations.

Remove some dead code

Improve visual diff code

Remove the gray background from annotation cards on hover.

 - Simplify a check for an error condition
 - Add a test case to avoid regressing the same error

As discussed in #1634

When creating a reply, there was an exception
on the console, because the target list is empty,
and some code was not prepared for that.

This takes care of that.

Fix #1661

[WIP] Restore feature to scroll for annotations

Reposition and style show differences checkbox

[WIP] Style the visual diff support

Clicking on an annotation card will bring (one of the) highlights
belonging to that annotation into the view in the document.

Restore visual diff to original behaviour.