Bump our Django Evolution dependency to 0.7.3.

This release includes fixes needed for upgrading from a Review Board
1.7.x or older release to Review Board 2.0.4+.
Include the revision in the "New Review Request" post-commit view.

This change adds the revision string to the list of commits in the post-commit
section of the "New Review Request" page. If the revision looks like a SHA-1
hash, we only show the first seven characters. Otherwise, we show the full
thing.

This also localizes the text in this view (I'm not sure how the "by ..."
slipped through).

Testing done:
Connected a github repo and opened the New Review Request page. Saw commit IDs.

Fixes bug 3402.

Reviewed at https://reviews.reviewboard.org/r/6140/
Show errors when trying to discard drafts.

The "Discard Draft" action would fail silently if something went wrong (for
example, if the server was inaccessible or the user was no longer logged in).
This change does some trivial plumbing to show the user an error in this case
(in the same way that we show other highly unexpected errors).

Testing done:
Loaded the page, killed my devserver, and tried to discard a draft. Saw an
alert that said "HTTP 0 error", which is at least better than nothing.

Sort-of addresses bug 3453.

Reviewed at https://reviews.reviewboard.org/r/6137/
Merge branch 'release-2.0.x'