Add Disabled mode to landing revision ui

Summary:
Fixes T4066.

add isActionDisabled() to DifferentialLandingStrategy, which also explains why it is so.
Make an appropriate pop-up in the controller.

Also make the whole UI "workflow", and convert createMenuItems() to createMenuItem() (Singular).

Test Plan: Click "Land to..." button in all kinds of revisions.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4066

Differential Revision: https://secure.phabricator.com/D8105