Modernize VCS password storage to use shared hash infrastructure

Summary: Fixes T4443. Plug VCS passwords into the shared key stretching. They don't use any real stretching now (I anticipated doing something like T4443 eventually) so we can just migrate them into stretching all at once.

Test Plan:

• Viewed VCS settings.
• Used VCS password after migration.
• Set VCS password.
• Upgraded VCS password by using it.
• Used VCS password some more.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4443

Differential Revision: https://secure.phabricator.com/D8272