Complete session digest migration from SHA1 to SHA256
Summary:
Followup to 1d34238dc94555466e15039ff6991b371ae294ef.
Crossing fingers that nobody complains about a one-time logout after their six year long user session.
Closes T16025
Test Plan:
* Log in, log out, do stuff.
* More specifically, with my session expired, successfully logging in created a database row added to `phabricator_user.phabricator_session` as expected, and logout removed the row.
Reviewers: O1 Blessed Committers, avivey
Reviewed By: O1 Blessed Committers, avivey
Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16025
Differential Revision: https://we.phorge.it/D25949