Summary:
Ref T4673.

IMPORTANT: I had to break one thing (see TODO) to get this working. Not sure how you want to deal with that. I might be able to put the element //inside// the workboard, or I could write some JS. But I figured I'd get feedback first.

General areas for improvement:

  - It would be nice to give you some feedback that you have a filter applied.
  - It would be nice to let you save and quickly select common filters.
  - These would probably both be covered by a dropdown menu instead of a button, but that's more JS than I want to sign up for right now.
  - Managing custom filters is also a significant amount of extra UI to build.
  - Also, maybe these filters should be sticky per-board? Or across all boards? Or have a "make this my default view"? I tend to dislike implicit stickiness.

Test Plan:
Before:

{F157543}

Apply Filter:

{F157544}

Filtered:

{F157545}

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: qgil, swisspol, epriestley

Maniphest Tasks: T4673

Differential Revision: https://secure.phabricator.com/D9211