R1:08966ba311cb

Begin modularizing fields in SearchEngines

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).