Clean up Space select/dropdown UI alignment

Summary: This alignment is off on edit forms, from the new overflow rules. Let's re-align everything for forms too.

Test Plan:
Add 2 new spaces, go to Maniphest, edit a task, see proper alignment of [Space] [Policy] view dropdowns. Check mobile alignment and Safari/FF.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17491