Fix "From" field in emails for --submit-as

When sending emails for review requests, reviews, and replies, we'd send it
from the request user, rather than the owner of the relevant object. This made
using --submit-as pretty ugly.

At some point in the future, we want to enable many users to collaborate on the
same review request, at which point we'll have to revisit this, but for now,
fix the bugs.

Testing done: Ran unit tests

Fixes bugs 1795 and 2500.
Reviewed at https://reviews.reviewboard.org/r/4418/
Add support for BitBucket's APIs, and support for Git.

We now use BitBucket's APIs for repository validation and for file
fetching. This, combined with usage of the new base_commit_id field,
allows us to finally add support for Git on BitBucket.

Unit tests were added to test all this functionality.
Set a sane default for hosting_service when validating repos.

We were accessing hosting_service without ensuring it always had a
value. We now default to None.
Merge branch 'release-1.7.x'
Improve i18n a bit.

I'm working on integrating the Italian localization that we recieved at
https://reviews.reviewboard.org/r/4298/

There are a few things that were getting in the way here. We have some old i18n
scripts in contrib/ which mostly work, but should be removed in favor of the
django-admin.py versions. The django-admin.py version of makemessages was
erroring out in a couple places where we were using positional format strings
with multiple arguments, instead of parameter names. I've fixed these up.

There are also several strings which aren't marked for translation. A lot of
these are likely coming from javascript, but one that was immediately obvious
was the dashboard's sidebar.

Testing done:
Integrated the contributed Italian localization files, and ran
compilemessages. Set my browser to prefer Italian, and saw that most of
the UI was now translated. Neat.

Reviewed at https://reviews.reviewboard.org/r/4426/