Add the option to use OpenStack Swift for file storage.

Swift is a private-cloud alternative to S3. This change adds backend support
and UI for using it for uploaded file storage.

Testing Done:
- Manual testing on local running server.
- Ran tests.

Reviewed at https://reviews.reviewboard.org/r/6247/
Fix file attachment URLs in change descriptions on local sites.

We didn't have the LocalSite name in the template, breaking links. This
fixes the templates to contain this information.

Reviewed at https://reviews.reviewboard.org/r/6262/
Remove the commit ID on a review request when discarded.

When posting a change for review with a commit ID and then discarding
it, that discarded review request still retains the commit ID. This
results in later conflicts with newly posted review requests, which
don't appear to break until the draft is published.

We now always remove the commit ID from the review request in order to
prevent any conflicts. While this is removing some history, it's history
that causes more harm than good and, since the review request is
discarded, it's not likely that the commit ID association is needed.
Merge branch 'release-2.0.x'