Summary:
Ref T8441. Ref T7715. Add a layer of indirection to fields in search engines. This will allow us to:
- Simplify SearchEngine code, which has collected a lot of duplication around expressing what is effectively field types.
- Automatically add fields like "Spaces" and "Projects" (primary driver for T8441).
- Reorder or hide fields (not sure if we really want to do this, but it seems plausible, and this will let us play around with it, at least).
- Drive Conduit Query methods via SearchEngines, so the same code specifies both the search UI and the `application.query` endpoint (primary driver in T7715).
Test Plan:
- Searched for stuff in Paste, everything behaved exaclty like it used to (except that I removed the "no language" checkbox, which seemed like fluff from a bygone era).
- Searched for stuff in other applications, saw no changes.
- Hit date field errors.
- Used query strings to specify values.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T7715, T8441
Differential Revision: https://secure.phabricator.com/D13169
