Allow PhabricatorUserLog to store non-user PHIDs

Summary:
Ref T4310. This is a small step toward separating out the session code so we can establish sessions for ExternalAccount and not just User.

Also fix an issue with strict MySQL and un-admin / un-disable from web UI.

Test Plan: Logged in, logged out, admined/de-admin'd user, added email address, checked user log for all those events.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4310

Differential Revision: https://secure.phabricator.com/D7953