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