Add 'Plans by cluster' view to projects page
Add 'Plans by cluster' view to projects page

Summary:
Cluster is becoming increasingly more relevant than master, and it's nice to
be able to view cluster associations across projects.

Screens: {F347314} {F347316}

Test Plan: Manual

Reviewers: paulruan

Reviewed By: paulruan

Subscribers: changesbot

Projects: #changes_ui

Differential Revision: https://tails.corp.dropbox.com/D180065

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