R1:c8574cf6fd74

Integrate ApplicationSearch with CustomField

Summary:
Ref T2625. Ref T3794. Ref T418. Ref T1703.

This is a more general version of D5278. It expands CustomField support to include real integration with ApplicationSearch.

Broadly, custom fields may elect to:

- build indicies when objects are updated;
- populate ApplicationSearch forms with new controls;
- read inputs entered into those controls out of the request; and
- apply constraints to search queries.

Some utility/helper stuff is provided…