CSS - add two rules to cover UI elements in really small spaces

Summary:
Fixes T7762.

@chad

noidea

...so feel free to reject immediately or otherwise tell me where / how these rules should live :D

Test Plan: Used Chrome and Firefox to test display on narrowest as possible widths. Looks better!

Reviewers: epriestley, chad

Reviewed By: chad

Subscribers: chad

Maniphest Tasks: T7762

Differential Revision: https://secure.phabricator.com/D12303