Simplify ".device-phone X, .device-tablet X { ... }" rules

Summary: Add a ".device" rule which means "phone or tablet". Simplify about 5000 rules which were written ".device-phone X, device-tablet X { ... }".

Test Plan: Browsed the site a bit without incident.

Reviewers: chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T1960

Differential Revision: https://secure.phabricator.com/D4103