Provide more storage space for password hashes and migrate existing hashes to "md5:"

Summary: Ref T4443. Provide more space; remove the hack-glue.

Test Plan: Logged out, logged in, inspected database.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4443

Differential Revision: https://secure.phabricator.com/D8269