The namespace filter will be built as part of T276261 but will only function in the JavaScript UI. This task covers handling the filter values as we have for other filters: adding the value(s) of this filter to the URL, respecting the value of the filter in the URL in the PHP UI, and showing a read-only version of the filter in the PHP UI.
Acceptance criteria:
[] When a user changes the value of the namespace filter, the value will be added to the URL
[] If All, Discussion, or General Help is selected, a matching keyword will be used as the URL param value
[] If Custom is selected, all of the selected namespaces must be added to the URL (either by key, e.g. NS_TALK, or by numerical ID)
[] The read-only filter will only show the keyword (including "custom") of the selected radio button, not all of the selected namespaces
[] Namespace filter settings (including custom namespace selections) are handled correctly during history navigation, disappearing when the user goes "back" and returning to the proper values when the user goes "forward" again.
[] URL parameters, UI state (checked boxes, radio buttons, etc) remain in sync at all times and always accurately reflect the namespace values being sent to the search API