Pop up auto-generated dialogs for Upload Diff and Upload Screenshot

Review Request #144 — Created Aug. 24, 2007 and submitted — Latest diff uploaded

Review Board SVN (deprecated)
Extended our dialog framework to support the creation of YUI dialogs based on Django Form objects. Submissions are handled asynchronously and error information is displayed on the form. When uploading, there's a spinner and "Uploading..." text so that the user knows something is happening.

The Upload Diff and Upload Screenshot links now pop up these dialogs, preventing the user from having to leave the page. The experience is much nicer.
Made sure I could upload diffs and screenshots. Put in lots of invalid data, left out fields, etc. and got the appropriate error messages.