Enable setting attachment thumbnails through the API.

The summary is a bit of a lie. This theoretically allows setting attachment
thumbnails through the API, but it's really just plumbing--it's up to individual
mimetype handlers to actually implement it (if they want).

Testing done:
Used this in conjunction with a review UI extension that takes advantage of it.
Allow users to remove ssh from admin settings.

This change adds a "Delete Key" button to the SSH key settings page, which
allows users to get rid of old keys.

Testing done:
Both manually and unit test passed, includes:
* Creating a key from scratch still works
* Deleting works

Manually tested:
* Clicking Save when there's a key, doesn't trigger any event

Reviewed at http://reviews.reviewboard.org/r/3758/
Fixes bug 2743
Convert the "set submitted" svn hook to new api.

The svn commit hook to set a review request submitted only worked with the
pre-1.6 api. This patch makes it work with the new api. It drops support for the
old api.

Testing done:
Tested on my private setup with reviewboard 1.7.4 and subversion 1.7

Reviewed at http://reviews.reviewboard.org/r/3909/
Enable usage of non-standard SSH ports for connecting to repositories.

This allows rbssh to use a non-standard port when given either the -p
parameter or when a port is in the provided UR (hostname:port).

Fixes bug #2841
Add a new UserSession model, and redo support for starring.

This introduces a UserSession model, which is a singleton that
represents the user's session. It is intended to replace gUsername, and
to provide the API for handling the watched groups/review requests.

The ReviewGroup model has been rewritten to be on Backbone.js and to
make use of this new API.

The existing ReviewRequest code now uses this API as well.

Unit tests were added to test the basic UserSession singleton logic, and
the individual setStarred functions.
Move the diff fragment loading into the new DiffViewer model.

This introduces RB.DiffViewer, which replaces and consolidates our old
diff fragment loading code. It cleans up the API a little and removes a
bunch of redundancy. It also simplifies RB.Diff, which was previously
doing too many different things.

This will later be used to store additional state of the diff viewer,
when that's ported over to use Backbone.
Port RB.Diff to Backbone.js.

The resulting RB.Diff is pretty tiny, since it's just a thin wrapper
over BaseResource's functionality now, and since the diff fragment
support has moved into DiffReviewable.

All it's used for now is for uploading new diffs. It provides an
rspNamespace and a function for returning human-readable error strings,
given an error payload.
Convert RB.Screenshot to Backbone.

This is a fairly trivial change to migrate the old screenshot object to
Backbone.js. The only tricky part of this is that I had to add an indirection
step to BaseResourceModel to ensure that the parent object is loaded before
calling url(), because if we don't have the parent links, we won't have a URL
for the object.

Testing done:
Tested setting screenshot captions and deleting screenshots. Ran js-tests.

Reviewed at http://reviews.reviewboard.org/r/3947/
Convert RB.ReviewReply to Backbone.

This is another relatively simple conversion for the data model for ReviewReply
objects to be based on BaseResource.

Testing done:
Replied to an existing review with body_top, body_bottom, and children
CommentReply objects. Published that reply and saw it get saved correctly.
Verified that loading a draft reply worked as expected. Ran js-tests.
Add a CONTRIBUTING guide.

GitHub will show this when someone's contributing. Hopefully it will
lessen the number of pull requests.
Merge branch 'release-1.7.x'