Harden repository passwords in the database.

Historically, passwords have been stored in plain text when directly
providing a repository not backed by a hosting service. Now, we store
this encrypted, like we do with any sensitive hosting service
information.

When upgrading the site, all passwords will be automatically migrated.

Reviewed at https://reviews.reviewboard.org/r/6400/
Release Review Board 2.0.9.
Merge branch 'release-2.0.x'