Don't allow forms which can't create objects to be added to profile menus

Summary:
Fixes T12281. Some forms (like Settings) can't actually create new objects. Currently, though, you can select them and add them to profile menus; if you do, they fail when building an item.

Kick them out of the typeahead, and decline to render them in menus.

Test Plan:
Added "Create Settings" to a menu, no longer fatals after patch (item vanished from menu, still editable normally to get rid of it).

Tried to add another "Create Settings", no longer available in typehaead.

Added some normal stuff.

Viewed a choose-among-forms dropdown in Maniphest, which still worked normally.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12281

Differential Revision: https://secure.phabricator.com/D17372