Add getStrList() to AphrontRequest

Summary:

• We have a few places where we do some kind of ad-hoc comma list tokenizing,

and I'm adding another one in D1290. Add a helper to the request object.

• Add some unit tests.

Test Plan:

• Ran unit tests.
• Used PHID manager, Maniphest custom view, and Repository project editor.

Reviewers: btrahan, fratrik, jungejason

Reviewed By: btrahan

CC: aran, btrahan, epriestley

Differential Revision: https://secure.phabricator.com/D1302