Share feed building code

Summary:
I pretty much copy/pasted this code; rather than do that again now that I want
to add feeds to projects, share the code.

This "Builder" is a little weird -- I don't want to call it a "View" because it
does data access. "Builder" seemed okay. We don't really have much code that
does this sort of thing right now, elsewhere.

Test Plan:

• Viewed public feed.
• Viewed private feed.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, btrahan

Maniphest Tasks: T681

Differential Revision: 1233