Make the paste creation form work on phones

Summary:
This is the first time I've ever had CSS actually work like it promises it does (i.e., markup the "right" way and then you don't have to change the markup later).

Since I laboriously laid this whole thing out with <divs> originally, I was able to just override some of the styles and make the layout reasonable for devices.

The only differences for existing forms are:

• No colon after labels (looks cleaner anyway).
• Non-error required text is no longer a red star but a the grey word "Required" (this is clearer).

Test Plan:
Viewed paste form on a phone.

Viewed ~20 other forms on the site to verify that I didn't break anything.

Reviewers: btrahan, chad, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1569

Differential Revision: https://secure.phabricator.com/D3298