Allow ApplicationEditor forms to be reconfigured

Summary:
Ref T9132. This diff doesn't do anything interesting, it just lays the groundwork for more interesting future diffs.

Broadly, the idea here is to let you create multiple views of each edit form. For example, we might create several different "Create Task" forms, like:

• "New Bug Report"
• "New Feature Request"

These would be views of the "Create Task" form, but with various adjustments:

• A form might have additional instructions ("how to file a good bug report").
• A form might have prefilled values for some fields (like particular projects, subscribers, or policies).
• A form might have some fields locked (so they can not be edited) or hidden.
• A form might have a different field order.
• A form might have a limited visibility policy, so only some users can access it.

This diff adds a new storage object (EditEngineConfiguration) to keep track of all those customizations and represent "a form which has been configured to look and work a certain way".

This doesn't let these configurations do anything useful/interesting, and you can't access them directly yet, it's just all the boring plumbing to enable more interesting behavior in the future.

Test Plan:
ApplicationEditor forms now let you manage available forms and edit the current form:

{F959025}

There's a new (bare bones) list of all available engines:

{F959030}

And if you jump into an engine, you can see all the forms for it:

{F959038}

The actual form configurations have standard detail/edit pages. The edit pages are themselves driven by ApplicationEditor, of course, so you can edit the form for editing forms.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9132

Differential Revision: https://secure.phabricator.com/D14453