Allow users to terminate login sessions

Summary:
This is partly a good feature, and partly should reduce false positives on HackerOne reporting things vaguely related to this.

Allow a user to terminate login sessions from the settings panel.

Test Plan:
  - Terminated a session.
  - Terminated all sessions.
  - Tried to terminate all sessions again.
  - Logged in with two browsers, terminated the other browser's session, reloaded, got kicked out.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D8556