R1:577020aea951

Move workboard "filter" workflow to a separate controller

Summary:
Depends on D20629. Ref T4900. Currently, the "Advanced Filter..." workflow on workboards (where you build a custom query) is inline in the main board controller.

This is because the filter flow depends on some of the board view state: we want to start with the current filter applied to the board, and preserve other state after you change the filter.

Now that `ViewState` can handle state management, we can separate this stuff out pretty…