R1:9a6c912ef6cb

Modernizing Flag Application

Summary:
Fixes a weird string in the flag dialog

Migrated to ObjectItemView (Cards)

Removed filters from side nav (unnecessary)

Go away, go away, panel. Nobody will miss you.

Adding sorting/ordering to Flag (separation like in Maniphest ordering still remaining)

Hacking our way in DifferentialRevisionListController thanks to panels (who added `->setFlush()` btw? It's awesome!)

Test Plan: I would not dare to stand here if it did not work.

Reviewers: epriestley, chad…