Currently, the namespace section of AdvancedSearch is always open. This makes AdvancedSearch use up more space than the "plain" search page, it increases loading time, and the namespace section slightly overshadows the "Advanced Parameters" section, so that it was sometimes not noticed.
Since not all searches run from Special:Search include users tinkering with namespace parameters, this all calls for a collapsing of the namespace section.
 There is a collapse/expand arrow in the top right corner of the namespace section (see mocks)
 By default, the namespace section is collapsed
 When the namespace section is collapsed, it shows pills with all active namespaces, like advanced search terms in the advanced parameters panel
 Pills in the collapsed view can be deleted by clicking on their "x", just like for pills of the advanced parameter's panel
The mocks also change the general look of the namespaces panel slightly (new titles and such). These are not part of this ticket, but !!TODO!!
This ticket only concerns itself with the redesign of the elements shown in the mocks above. Performance optimizations are to be taken care of in a separate ticket.