Ref T13043. Migrate VCS passwords away from their dedicated table to new the new shared infrastructure.

Future changes will migrate account passwords and remove the old table.

• Ran migrations.
  • Cloned with the same password that was configured before the migrations (worked).
  • Cloned with a different, invalid password (failed).
• Changed password.
  • Cloned with old password (failed).
  • Cloned with new password (worked).
• Deleted password in web UI.
  • Cloned with old password (failed).
• Set password to the same password as it currently is set to (worked, no "unique" collision).
• Set password to account password. This (incorrectly) works for now until account passwords migrate, since the uniqueness check can't see them yet.
• Set password to a new unique password.
  • Cloned (worked).
  • Revoked the password with bin/auth revoke.
  • Verified web UI shows "no password set".
  • Verified that pull no longer works.
  • Verified that I can no longer select the revoked password.
• Verified that accounts do not interact:
  • Tried to set account B to account A's password (worked).
  • Tried to set account B to a password revoked on account A (worked).
• Spot checked the password and passwordtransaction tables for saniity.