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.
• Viewed user profile / project profile feeds.
• Used function tokens (viewer(), etc).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12762

Differential Revision: https://secure.phabricator.com/D18028