Add a basic remarkup typeahead for users and projects

Summary: Ref T3725. This probably has 900,000 bugs. This will need updates for subprojects/milestones.

Test Plan:

• Tested very gently in Safari, Firefox and Chrome.
• Reasonable inputs appear to work.
• Clicking, escape, tab, return, arrow keys work OK?

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T3725

Differential Revision: https://secure.phabricator.com/D15029