[Redesign] Center Pinboard Items
Summary: Ref T8099, This centers, but floats, all pinboards items based on screen width.
Test Plan:
Test desktop, mobile break points. Items scale nicely.
Reviewers: btrahan, epriestley
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T8099
Differential Revision: https://secure.phabricator.com/D13085