Allow non-authors to "Request Review" of draft revisions

Summary:
See PHI1810. In situations where:

• An author submits an urgent change for review.
• The author pings reviewers to ask them to look at it.

...the reviewers may not be able to move the review forward if the review is currently a "Draft". They can only "Commandeer" or ask the author to "Request Review" as ways forward.

Although I'm hesitant to support review actions (particularly, "Accept") on draft revisions, I think there's no harm in allowing reviewers to skip tests and promote the revision out of draft as an explicit action.

Additionally, lightly specialize some of the transaction strings to distinguish between "request review from draft" and other state transitions.

Test Plan:

• As an author, used "Request Review" to promote a draft and to return a change to reviewers for consideration. These behaviors are unchanged, except "promote a draft" has different timeline text.
• As a non-author, used "Begin Review" to promote a draft.

Differential Revision: https://secure.phabricator.com/D21403