Rename Differential field Projects to Tags

Summary: Users can't find the "Tags" field in the Edit Menu; Added keyword "Tag".

Test Plan: Looked in Edit page; I think this shouldn't change anything else?

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15749