R1:6fed08a98e04

Modernize FeedSearchEngine a little bit

Summary:
Ref T12762. This updates FeedSeachEngine to user modern construction.

I've tried to retain behavior exactly, although the "Include stories about projects I'm a member of" checkbox is now nonstandard/obsolete. We could likely fold that into "Include Projects" in a future change which does a backward compatibility break.

Test Plan:
- Queried feed without constraints.
- Queried feed by user, project, "stuff I'm a member of", prebuilt "Tags" query.