Introduce ReleephProjectQuery

Summary:
Adds a policy-aware query class for selecting Releeph projects. This doesn't really change anything.

• Make ReleephProject implment PhabricatorPolicyInterface, beginning the long journey to make it policy-aware.
• Implement ReleephProjectQuery, for querying projects using cursor-based, policy-aware paging.
• Use it on the list view, so we load only ~100 projects instead of all of them.
• Tweaked some of the URI routing stuff to make it a little more consistent with common practices.

Ref T2714.

Test Plan:
{F36434}
{F36435}

Reviewers: edward

Reviewed By: edward

CC: aran

Maniphest Tasks: T2714

Differential Revision: https://secure.phabricator.com/D5390