Allow configuration to have custom UI types

Summary:
Ref T1703. This sets the stage for (but does not yet implement) custom UI types for config. In particular, a draggable list for custom fields.

I might make all the builtin types go through this at some point too, but don't really want to bother for the moment. It would be very slightly cleaner but woudn't get us much of anything.

Test Plan:
UI now renders via custom code, although that code does nothing (produces an unadorned text field):

{F45693}

Reviewers: chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T1703

Differential Revision: https://secure.phabricator.com/D6154