Move the repository policy step into the create workflow

Summary:
Fixes T4242. It's currently possible to set nonsense defaults and create repositories with unintended policies, because policy configuration isn't part of creation. Instead:

• put a policy page into the creation workflow;
• require the selection of valid policies (i.e., prevent creating a repository you can't view / edit).

Test Plan:

• Created imported and hosted repositories, hit policy selection.
• Edited policies of existing repositories.
• Tried to set nonsense policies.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4242

Differential Revision: https://secure.phabricator.com/D7856