Add support for branch-related configuration to new Repository edit workflow

Summary: Ref T2231. Modernizes editing "Default Branch", "Track Only", and "Autoclose Only".

Test Plan: See screenshots.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2231

Differential Revision: https://secure.phabricator.com/D7399