Build an early multi-step repository create form

Summary: Ref T2231. Ref T2232. This form doesn't do anything yet and there are no link sto it, but it lets you enter all the data to create a repository in a relatively simple, straightforward way.

Test Plan:
{F49740}
{F49741}
{F49742}
{F49743}

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2231, T2232

Differential Revision: https://secure.phabricator.com/D6430