Tweaking Widget Panel in Conpherence

Summary:
Fixes T3252. Other enhancements:

- Header in widget panel was 2px too short.
- Typeahead in add people only allowed one person
- Typeahead in add people was cutoff by overflow:hidden
- X in remove has been changed to unicode (multiply)
- Add people dialog form fields are full width
- Some other CSS tweaks.

Test Plan: Add, Remove people.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3252

Differential Revision: https://secure.phabricator.com/D6076