Widen pinboard to 280x210.

Summary: This widens pinboard images to 280x210, which neatly fits on an iPhone 4, and gives more visual space to Macros and Mocks.

Test Plan: Test Pinboard in Chrome and iOS simulator.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5224