Clean up Maniphest Batch Editor UI

Summary: Finishes up work on PHUIInsetForm, cleans up the UI a bit when editing multiple tasks

Test Plan:
Edit multiple tasks, apply some new projects, still works as expected.

{F236044}

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D10881