R1:eef314b70149

Separate session management from PhabricatorUser

Summary: Ref T4310. Ref T3720. Session operations are currently part of PhabricatorUser. This is more tightly coupled than needbe, and makes it difficult to establish login sessions for non-users. Move all the session management code to a `SessionEngine`.

Test Plan:
- Viewed sessions.
- Regenerated Conduit certificate.
- Verified Conduit sessions were destroyed.
- Logged out.
- Logged in.
- Ran conduit commands.
- Viewed sessions again.