Added search for all lists on issue boards
## What does this MR do?
Adds a search box to allow the user to search all lists in issue boards rather than just the backlog.
## Screenshots (if relevant)
![Screen_Shot_2016-08-30_at_10.33.15](/uploads/67e96055d60a9b3209ce3831a1980c09/Screen_Shot_2016-08-30_at_10.33.15.png)
![Screen_Shot_2016-08-30_at_10.33.19](/uploads/8d5253b8f2ecf1cf9a69d70be8ccf1f9/Screen_Shot_2016-08-30_at_10.33.19.png)
## What are the relevant issue numbers?
Closes#21139
See merge request !6101