Upstream task about sub-projects: https://secure.phabricator.com/T3670
As someone who monitors a workboard for a large team that works on many projects, I would like to be able to easily narrow my focus to the activity of a particular project. Even though I can filter the project, I would like to have some assurance that the workboard for the team is in sync with the workboard for the project, if it exists. For example, as of this writing, the workboard for the "MediaWiki Core Team" filtered to only show CirrusSearch tasks is out of sync with the CirrusSearch workboard.
One solution is to make it possible to designate a project's workboard as a sub-board of another board. So, in the case of CirrusSearch, it would be possible to designate that to be a sub-board of MediaWiki Core, so that the view in the board would be equivalent to if the MediaWiki Core board was filtered to only show CirrusSearch tasks. The tasks that only exist in CirrusSearch and not in the MediaWiki Core parent would ideally still show up on the CirrusSearch board (thus being different than a mere filtered view of the parent board), but wouldn't be expected to show up in the parent board.