Switch over to file attachments entirely in place of screenshot uploading.

This removes the ability to upload screenshots, and instead moves to
using file attachments for everything.

Dragging-and-dropping a file over the review request will just have a
big "Drop to Upload" link, instead of "Drop Screenshot" and
"Drop File Attachment."

The "Upload Screenshot" link on a review request is gone.

The entry for a non-repository review request now says
"(None - File attachments only)" instead of "(None - Graphics only)".
Show the thumbnail for a file attachment after uploading to a review request.

When uploading a file attachment onto a review request, we weren't
showing the thumbnail until we reloaded the page. That data was
available to us as part of the API, we just weren't using it. Now we set
the thumbnail when building the file attachment display.
Immediately display the reply draft banner after adding a comment.

The reply draft banner wasn't first showing up until after scrolling or
resizing the window. This was due to the banner not having a container
until later in the call stack. To work around this, we just need to
defer the initial call to update the position until the call stack
unwinds, since we'll then have a container and can make the proper
computations.
Set a tracking branch for commits on origin/release-1.7.x.
HostingServices can check if SSH keys are associated

Automatic SSH key association should only be performed if the key has not
already been associated. HostingServices that support SSH key association now
have the capability to report if a given key has been associated with a given
repository. Currently, only the GitHub HostingService supports SSH key
association.

Added unit test for code that checks if a key has been associated. Made slight
adjustment to key association test to ensure tests pass (given that a helper
method for SSH key association API calls was added). Both unit tests pass.

Tested manually with an existing GitHub repository by:

1) Adding RB server's (dev server's) SSH key to list of repository deploy keys.
   Check confirmed key was associated.
2) Adding SSH key to list of user keys. Check confirmed key was associated.
3) Ensuring key NOT associated with repository. Check confirmed key NOT
   associated.
4) Ensured associate_ssh_key still works as normal for GitHub.

Reviewed at http://reviews.reviewboard.org/r/3478/
Fix a regression with updating extra comment data.

The newly introduced _import_extra_data wasn't handling deletion of data
very well, and this broke unit tests.
Fix the cache stats page.

The cache stats page hasn't really worked correctly in a while. This may
be due to the Django changes to cache backends. Either way, we're now
working properly with cache backends, and format our content in a way
that's consistent with the admin UI.
Fix editing of draft captions for screenshots.

A class name was updated that shouldn't have been for the screenshot
thumbnails. This prevented our inlineEditors from being registered, and
broke screenshot caption editing.
Add release notes for 1.6.14.
Actually add the release notes.

These got left out of my prior commit.
Fix the Server setting field being blank.

Due to a change made in Djblets at one point, the Server setting field
is always blank. We need to set the loaded value after we've loaded the
base form, not before, or it'll get overwritten.

Fixes bug #2757
Require Djblets 0.6.27.
Fix pyflakes warnings.

None were harmful. Just unused imports and variables.
Merge branch 'release-1.6.x' into release-1.7.x
Link to the 1.7 docs from the navbar's Documentation link.
Merge branch 'release-1.7.x'