Version the Aphlict notification server and prompt users to upgrade if they're out of date

Summary: Ref T4324. Add some version information to the server status output, and setup checks to test for an unreachable or out-of-date server.

Test Plan:
  - With server down, hit reasonable setup check.
  - With server up and at a bad version, hit reasonable setup check.
  - Viewed `/notification/status/`.
  - The CSS thing fixes this:

{F114445}

Reviewers: btrahan, chad

Reviewed By: chad

CC: chad, aran

Maniphest Tasks: T4324

Differential Revision: https://secure.phabricator.com/D8251