R1:c9d3fb2ac5b6

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…