Partially support CustomFields in EditEngine

Summary:
Ref T9132. This isn't perfect, but doesn't break any existing functionality. This stuff works:

• Editing values.
• Reordering fields.
• All builtin field tyepes.

This stuff may not work yet:

• Assigning custom field defaults.
• Some conduit stuff.
• Fully custom fields?
• Locking/hiding fields? Didn't actually test this one.

I'll keep chipping away at that stuff. In some cases, it may be easier to convert all the CustomField apps first, although Differential might be a fair bit of work.

Test Plan:
Created a bunch of custom fields of every avialable type and edited them.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9132

Differential Revision: https://secure.phabricator.com/D14617