Add UI for defining repository mirrors

Summary:
Ref T4038. This adds everything except the actual pushing part for mirrors.

This isn't the most beautiful or sophisticated UI, but I want get the authoritative repositories self-hosted and get users beta-ing hosting as soon as possible. We can do transactions, etc., later on.

Test Plan: See screenshots.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4038

Differential Revision: https://secure.phabricator.com/D7632