Implementation of VCS passwords against user.

Summary: This allows users to set their HTTP access passwords via Diffusion interface.

Test Plan: Clicked the "Set HTTP Access Password" link, set a password and saw it appear in the DB.

Reviewers: #blessed_reviewers, hach-que, btrahan

Reviewed By: hach-que

CC: Korvin, epriestley, aran, jamesr

Maniphest Tasks: T2230

Differential Revision: https://secure.phabricator.com/D7462