[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