Simplify annotation component by removing duplicated state

Remove site scripts and assets

The correct fix would be to remove new annotations with an empty draft
in the reducer function that handles new annotations being created. That
is not currently possible because drafts are not stored in the Redux
store. Therefore this commit just manually de-registers the listener
when <annotation> goes out of scope.

The `updateViewModel()` function was responsible for computing several
pieces of data (links, document domain and title) used by the annotation
template.

Since we can now rely on vm.annotation being immutable, we can just
compute this data lazily when we need it and use memoization to avoid
unnecessary recalculation.

Replace `updateDomainModel()` with a simpler function which returns a
copy of the input annotation with the changes from the editor applied.

These variables duplicated the information from the current draft for an
annotation. Instead of restoring a draft when an annotation card is
created and saving it when the annotation card is destroyed, just update
the draft whenever the tags, text or privacy of the annotation is
changed. The text/tags/privacy shown in the card are determined by
`vm.state()` which merges the annotation and the draft.

Removing these variables lets us also remove or simplify the tests that
checked that everything remained in sync.

Instead of maintaining a state flag indicating whether an annotation is
being viewed, edited or created, derive that state from whether the
annotation has an ID and whether it has an unsaved draft or not.

In the process this commit simplifies the tests for reverting edits and
adds a missing test that the annotation is deleted if new when clicking
the Cancel button.

As a step towards making <annotation> stateless or nearly-stateless and
instead just a display of an annotation derived from the current
annotation instance and its draft, remove the `domainModel` copy of
`vm.annotation`.

Make `gulp watch` build CSS, files and images

Make `gulp watch` build the CSS, files and images initially (creating
the output files if they don't exist yet, overwriting them if they do)
and then watch the source files and re-build them when they change.

This matches the existing `gulp watch` behaviour for scripts.

This means you can just run `gulp watch` and have everything built,
instead of having to run `gulp build watch`.

Fix annotation state not being updated when annotations are saved

Annotations in the app's local state store were not being updated
after a save was successfully committed on the server.

Fixes hypothesis/h#2965

zen-observable update

Remove use of ngResource for search and annotation queries

Show direct links on stream page when available (FIRST!)

The CHANGES file is part of the 'h' repo and no longer exists in the
'client' repo that was forked from it.

Move relevant tests to annotation-metadata-test.js, and fix a bug
in annotation-metadata.js.

Several aspects of ngResource make it sub-optimal for our needs:

 1. It mutates the model object directly after making an API call, which
    does not fit well with usage in a Redux app where the UI state should
    be an immutable object.

 2. The ngResource classes can only be constructed once the API
    description has been retrieved. At least one place in our code,
    which handled newly created annotations arriving from the page,
    failed to account for this.

This commit therefore replaces use of ngResource for making API calls
to the search and annotation endpoints with a simple wrapper around HTTP
requests which makes a call and then returns the resulting object.

The new wrapper will wait until the API description has been received
before attempting to make an API call, avoiding problem (2) above.

As a bonus, this brings us a step towards decoupling the JS API client
from Angular so we could re-use it outside the app in future.

 * Replace ngResource with simple $http wrapper in store.js

 * Add tests for annotation update and deletion API calls

 * Change tests in annotation-test.js from
   `assert(predicate(actual, expected))` form to
   `assert.predicate(actual, expected))` form as this results in errors
   that are _much_ easier to debug when they fail.

The primary change here is an update for zen-observable.

Update to zen-observable v0.3.0 which implements several changes to the
spec, the most notable of which is that Observable.{of, from, forEach}
are now all synchronous [1].

To preserve the previous behavior of `selections()` where no events were
emitted until the next tick of the event loop, add a delay before
checking for the initial selection.

[1] https://github.com/zenparsing/es-observable/issues/88

Trigger Angular digest cycle if necessary during Redux action dispatches

This is no longer needed following the addition of middleware to the
Redux store which triggers a digest following a UI state change if
necessary.

Also add a comment in the WebSocket code that we will be able to remove
the $apply() call once session state has been moved to the Redux store.

At various places in our code that receive external inputs (eg.
WebSocket messages, messages from other frames etc.) we have to wrap
that handling in a $scope.$apply() call or similar in order to trigger
Angular's change detection cycle and sync UI components with the updated
state of the app after handling that input.

Since most changes to app state now happen as a result of a Redux action
being dispatched, we can use Redux' built-in middleware mechanism for
adding logic around app state changes.

Improve behavior when selecting pre-created annotations

Selecting a pre-created annotation in the page resulted in a
'You do not have permission to see this annotation' being displayed
because the selection mechanism only supports annotations which have a
server-assigned ID.

This commit improves that behavior by ignoring annotations without IDs
when selecting annotations.

It also fixes an issue in AnnotationUISync that prevents Annotation
objects being replaced in the UI state after they are updated on the
server. Previously AnnotationUISync would receive the tags of
annotations that had been selected in the page and would then look up
annotations in a cache maintained by AnnotationSync in order to get
Annotation objects that could be passed to `annotationUI` functions
which needed objects with an ID.

Fixes #3547