R1:4b7f3b709d0d

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…