Move workboard "filter" workflow to a separate 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 easily.
- Changed filters on a board.
- Applied a custom filter to a board.
- Changed the ordering of a board, then applied a custom filter. Verified "Cancel" and "Apply Filter" both preserve the order state.
- Changed the ordering of a board, then applied a custom filter, intentionally making a mistake in configuring the filter by entering an invalid date. Saw a dialog with an error. After correcting the error, saw state preserved properly.
Reviewed By: amckinley
Maniphest Tasks: T4900
Differential Revision: https://secure.phabricator.com/D20632