R1:b85f8b91de69

Implement readProjectsFromRequest() helper for SearchEngines

Summary:
Ref T4100. This just makes the "specify stuff in query parameters" workflow a little better:

- You can now do `?projects=differential,diffusion`.
- You can now do `?projects=projects(alincoln)`.

Test Plan: Did that stuff ^^^^

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4100

Differential Revision: https://secure.phabricator.com/D12468