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 to make this easier. This part could be cleaner, but seems reasonable for a first cut. In particular, the Query and SearchEngine must manually call all the hooks right now instead of everything happening magically. I think that's fine for the moment; they're pretty easy to get right.

Test Plan:
I added a new searchable "Company" field to People:

{F58229}

This also cleaned up the disable/reorder view a little bit:

{F58230}

As it did before, this field appears on the edit screen:

{F58231}

However, because it has `search`, it also appears on the search screen:

{F58232}

When queried, it returns the expected results:

{F58233}

And the actually good bit of all this is that the query can take advantage of indexes:

  mysql> explain SELECT * FROM `user` user JOIN `user_customfieldstringindex` `appsearch_0` ON `appsearch_0`.objectPHID = user.phid AND `appsearch_0`.indexKey = 'mk3Ndy476ge6' AND `appsearch_0`.indexValue IN ('phacility') ORDER BY user.id DESC LIMIT 101;
  +----+-------------+-------------+--------+-------------------+----------+---------+------------------------------------------+------+----------------------------------------------+
  | id | select_type | table       | type   | possible_keys     | key      | key_len | ref                                      | rows | Extra                                        |
  +----+-------------+-------------+--------+-------------------+----------+---------+------------------------------------------+------+----------------------------------------------+
  |  1 | SIMPLE      | appsearch_0 | ref    | key_join,key_find | key_find | 232     | const,const                              |    1 | Using where; Using temporary; Using filesort |
  |  1 | SIMPLE      | user        | eq_ref | phid              | phid     | 194     | phabricator2_user.appsearch_0.objectPHID |    1 |                                              |
  +----+-------------+-------------+--------+-------------------+----------+---------+------------------------------------------+------+----------------------------------------------+
  2 rows in set (0.00 sec)

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T418, T1703, T2625, T3794

Differential Revision: https://secure.phabricator.com/D6992