Modernize Releeph branch edit and create interfaces

Summary: Ref T3092. Fixes T3724. Use modern/flexible UI for these interfaces. Removes the ability to retarget an existing branch (you can just close it and open a new one if you made a mistake).

Test Plan: {F54437} {F54438}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3092, T3724

Differential Revision: https://secure.phabricator.com/D6765