Add an "active login sessions" table to Settings

Summary: Ref T4310. Ref T3720. Partly, this makes it easier for users to understand login sessions. Partly, it makes it easier for me to make changes to login sessions for T4310 / T3720 without messing anything up.

Test Plan: {F101512}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3720, T4310

Differential Revision: https://secure.phabricator.com/D7954