Add subvertpy as an alternative to PySVN

"subvertpy" is a new python library for subversion which has the distinct
advantage of being installable with easy_install. This change generalizes the
SVN SCMTool to use either subvertpy or PySVN, depending on which one is
installed.

Testing Done:
* Made sure there were no regressions in the unit testsuite (Python 2.7,
  Linux), and made sure that PySVN still works.
* Light manual testing (Python 2.7, Linux).

Reviewed at https://reviews.reviewboard.org/r/5104/
Mark quick ship-it reviews as rich text.

This doesn't actually have any effect on normal quick ship-its, but will make
the ship-it ascii art extension nicer.

Reviewed at https://reviews.reviewboard.org/r/5295/
Make "Auto-Submitted: auto-generated" header optional with a settings flag

Some mailing lists filter out emails containing the "Auto-Submitted:
auto-generated" header. Making this header optional allows those who use such
mailing lists to receive Review Board emails.

Testing Done:
* Manually enabled and disabled the header settings flag in E-Mail Settings.
* Ran unit tests.

Bugs closed: 2646

Reviewed at https://reviews.reviewboard.org/r/5276/
Show discarded review requests in the "All Review Requests" view.

For some reason, we chose to filter public discarded review requests out of the
list view. This made it very hard to find old changes that had been closed out
for some reason (they were only available in the submitter lists). This change
includes them along with "Submitted" review requests, and changes the show/hide
toggle to be "closed" instead of "submitted".

Testing done:
Discarded a review request. Saw that I could see it in the "All Review
Requests" view. Checked that "Hide closed" filtered out both submitted and
discarded review requests.

Fixes bug 576.

Reviewed at https://reviews.reviewboard.org/r/5303/
Abbreviate SHA-1 commit IDs.

If a review request is created with a SHA-1 as its commit ID (for example, when
using post-commit for GitHub), the long string of text will push everything
over, making the UI very ugly.

This change adds some code to abbreviate 40-character strings.

Testing done:
Created a post-commit change and saw that it showed me an abbreviated SHA
instead of the full thing.

Reviewed at https://reviews.reviewboard.org/r/5304/
Convert review group's mailing list to a generic CharField.

Our email backend allows the mailing list field to contain multiple e-mail
addresses separated by commas. Unfortunately, because this was represented as
an EmailField, the validation in the admin UI didn't allow entering multiple
addresses. This change converts the field to be a generic CharField.

Testing done:
* Ran evolution
* Ran unit tests
* Added a group with multiple mailing lists.

Fixes bug 1661.

Reviewed at https://reviews.reviewboard.org/r/5305/
Split accounts.forms into new modules.

While working on the new My Account page, I hit some circular dependency
issues with forms and backends. These weren't new, it turns out, as we
had sprinkled imports in the PreferencesForm functions.

Rather than adding more of that, I decided to split this file up.
There's now three modules: account (for account-related forms),
registration, and auth (for auth backends).