Make "Land Revision" button state consistent, prevent non-accepted lands

Summary:
Ref T182. Make the disabled state of the button more accurately reflect whether clicking it will work.

Don't allow "land" to proceed unless the revision is accepted.

Test Plan: Saw button in disabled state, clicked it, got "only accepted revisions" message.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T182

Differential Revision: https://secure.phabricator.com/D14350