Motivation
Currently, the namespace selection and deselection happen at two different points in the interface, which is pretty cumbersome. We want to allow users to select and deselect from the same place.

Acceptance Criteria
[] Instead of the namespace icons, namespaces are now preceded by a checkbox. Clicking on a line of an unselected namespace, it becomes selected. This means the checkbox is checked and the background is is highlighted as usual in OOUI standards. Clicking on the line of a selected namespace, the inverse happens (please check the recent changes namespace filter for the exact highlighting behavior (because it's a bit weird with being white when nothing is selected and turning gray except for the selected things the instance something gets selected))

• There is a visible line grey line differentiating content + talk pages from each other
• If a namespace is selected, its name appears as a pill on top of the input bar

• Add a OOUI magnifying glass in the input bar to make it more visible that people can search for namespaces

[] As before, when users enter the input bar, the dropdown expands. When they start typing, the dropdown reduces to the suitable options