Move generation of the namespaces pills from the filter box (where you can type) to under the presets (General, Discussion, etc.)
See [[ https://phab.wmfusercontent.org/file/data/47rqleoeg6lestpqpxiv/PHID-FILE-2otgprffl47bqz23ogta/Bildschirmfoto_2018-08-21_um_12.01.28.png | mock ]]
Acceptance Criteria:
[] Namespace pills appear under the namespace presets as shown in the mock
[] SearchableCheckboxWidget from {{T202505}} is displayed
Background: T202370
Note: Our custom `NamespaceFilters` widget can no longer use `MenuTagMultiselectWidget`. Instead, it should use `TagMultiselectWidget`. There are two possibilities to integrate the new widget:
a) with the `inputWidget` property of `TagMultiselectWidget`. Have a look at how the menu widget and its select events inside `MenuTagMultiselectWidget` are integrated with the `TagMultiselectWidget`
b) as a standalone widget below the `TagMultiselectWidget`. With this solution, there is no need for the widgets to listen to each other, their state is shared and updated though the SearchModel. However we'd have to find out if `TagMultiselectWidget` can be used without any input widget facility