Currently, Review Board's diffs can only be viewed within Review

Board. Sometimes, however, it would be nice to take a diff and

show it on another webpage, muchlike you can with GitHub's Gists

service, or a YouTube video.
This change creates an endpoint for the individual FileDiffs

from the DiffViewer for an iframe to point to. E.g.:
reviews.reviewboard.org/r/(review_request_id)/diff/(revision)/embed/(filediff_id)/
When this endpoint is hit, the necessary files to display a single

Diff fragment are returned as if it were extracted from the DiffViewer.
This change also includes a chain link icon beside the filename

in the diff viewer which, when clicked, shows a box with

pre-filled input fields which will hold embed and permalinks to

that file diff.
The full URL is generated and placed in the diff viewer dropdown box

Previously, the path was only included and now the full URL, with

the HTTP host, is included.
A new template has been created to display a diff fragment instead

of using the diff fragment template from the diff viewer. This will

save on loading more static files than necessary for the embedded

diff fragment and provides more control.
The proper styles have been applied to the diff fragment embed so

the embedded diff now looks presentable and in-line with the RB

theme.
The original diff fragment view and the diff fragment embed view

shared a lot of the same code. Similar code between these two

class based views were extracted into a parent class.
A link was also added to the embedded diff fragment that easily

allows a user to view the full diff on review board.

Tested URI in browser with no ill side effects.
Embed box needs to be fixed in Safari. Timeout is too fast.