Ability to add reviewers while requesting review

Summary: It makes perfect sense to add more reviewers while requesting review.

Test Plan:
Request review. Verify that Add Reviewers field shows and works.
Add some reviewer. Verify that comment preview works.
Submit. Verify that reviewers are saved and displayed.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1473