Allow EditEngine to build NUX buttons that point at the right place

Summary:
Fixes T11812.

• Pull the logic for building the "Create Whatever" dropdown out.
• Use it to generate NUX buttons, too.
• Use the new logic in Paste and Maniphest.

Test Plan:

• Viewed Paste NUX, button worked.
• Viewed Maniphest NUX with multiple create forms, button worked.

Reviewers: chad, avivey

Reviewed By: avivey

Subscribers: avivey

Maniphest Tasks: T11812

Differential Revision: https://secure.phabricator.com/D16797