Depends on D20632. Ref T4900. As with other workflows on the board controller, this one is currently in the giant main "do everything" method. Move it to a separate controller.

This makes one material improvement: previously, we built the full board and did layout on all the cards before building the query. However, we do not actually need to do this: we don't need the cards. Instead, just do layout without handing over any card PHIDs. This is slightly faster, particularly on large boards.

• Clicked "View as Query" on a board, got a query page for the column.
• Applied a custom filter, then clicked "View as Query" on a board. Got a query page merging the two filters.
• Applied a custom filter, then clicked "Veiw as Query" on a board, in a subproject column. Got a query page merging the two filters, respecting the project-ness of the column.