Allow plus signs in usernames; update docs.

Testing Done:
All unit tests pass except perforce-related ones, which were skipped, since I
don't have it installed.

Reviewed at https://reviews.reviewboard.org/r/5036/
Merge branch 'release-1.7.x'
wrap 'Latest diff' in {% blocktrans %} like the blocks above
Merge remote-tracking branch 'upstream/master'