Move activity log rendering to a dashboard panel

Summary:
Ref T4986. Swap this in. Two minor notes:

• I adjusted the SearchEngine to add an additional constraint when the viewer isn't an admin. This mostly stops us from doing a bunch of unnecessary work.
• I fixed the settings panel to paginate (currently loads all results, slow in production).

Test Plan: Viewed logs; viewed settings panel; created a dashboard panel.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4986

Differential Revision: https://secure.phabricator.com/D9136