Make the 'Subscribe' button pop a modal instead.

Summary:
See title.

Also some minor styling/consistency fixes.

Test Plan:

• Clicked subscribe
• Canceled to make sure it went away
• Clicked it again
• Clicked subscribe
• Saw my name in the cc field.

Reviewers: epriestley, chad, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4571