Improve user feedback when no issues are on Rapid Board

As a user, I would like to have a consistent, and accurate message displayed when I have configured my rapid board to show no issues. Examples:

1. My quick filter results in nothing appearing in the backlog. Currently, the message shown is: "There are currently no issues in the backlog" This should be improved to say: "There are currently no issues in the backlog which match your quick filter."

2. I select an Epic with no sub issues. Currently, there is no message shown at all. This message should be added to meet the same format: "There are currently no issues in the backlog which match your selected Epic."

Another possible implementation method would be to take the WIP issue count from Kanban boards and apply it to the backlog as well. Screenshot of this attached. Though telling a user what is causing them to be hidden may also be helpful.