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
Always include the current user as a selectable policy

Summary: Ref T4136. After Passphrase, user policies work correctly in this dropdown. Providing this option improves consistency and makes it easier to create, e.g., a private repository (where "no one" does not include the viewer, because they don't own the resulting object).

Test Plan: Set an object's policy to my user policy.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4136

Differential Revision: https://secure.phabricator.com/D7858