When creating a repository in Diffusion, prompt for "Create" or "Import" first

Summary:
Ref T2230. This will need some more refinement, but basically it adds a "Create" vs "Import" step before we go through the paged workflow.

  - If you choose "Create", we skip the remote URI / auth stuff, and then set the "hosted" flag.
  - If you choose "Import", we do what we do now.

Test Plan: Created and imported repos.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2230

Differential Revision: https://secure.phabricator.com/D7475