Allow PhabricatorUserLog to store non-user PHIDs

Summary:
Ref T4310. This is a small step toward separating out the session code so we can establish sessions for `ExternalAccount` and not just `User`.

Also fix an issue with strict MySQL and un-admin / un-disable from web UI.

Test Plan: Logged in, logged out, admined/de-admin'd user, added email address, checked user log for all those events.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4310

Differential Revision: https://secure.phabricator.com/D7953
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
Icons for TagView

Summary: Adds the ability to set icons into Tags.

Test Plan: tested on UIExamples page.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7961
Merge remote-tracking branch 'upstream/master' into release