Migrate repositories to use Passphrase for credential management

Summary: Fixes T4122. Ref T2230. Instead of storing credentials on each repository, store them in Passphrase. This allows easy creation/management of many repositories which share credentials.

Test Plan:
  - Upgraded repositories.
  - Created and edited repositories.
  - Pulled HTTP and SSH repositories.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2230, T4122

Differential Revision: https://secure.phabricator.com/D7629