Added search for all lists on issue boards

Adds a search box to allow the user to search all lists in issue boards rather than just the backlog.

![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)

Closes #21139

See merge request !6101
Conflicts:
	app/assets/stylesheets/pages/boards.scss
	app/views/projects/boards/components/_board.html.haml