Fix the incorrect link target for "Create Revision" as a Menu Item

Summary:
Depends on D20359. Fixes T12098. When you add a new "Form" item and pick "Create Revision", you currently get a bad link. This is because Differential is kind of special and the form isn't usable directly, even though Differential does use EditEngine.

Allow EditEngine to specify a different create URI, then specify the web UI paste-a-diff flow to fix this.

Test Plan:

• Added "Create Revision" to a portal, clicked it, was sensibly put on the diff flow.
• Grepped for getCreateURI(), the only other real use case is to render the "Create X" dropdowns in the upper right.
  • Clicked one of those, still worked great.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12098

Differential Revision: https://secure.phabricator.com/D20360