Create SearchableCheckboxWidget to replace the menu with the icons in the namespaces dropdown part of AdvancedSearch.

Acceptance Criteria:

• Initially, only the search input field (and its icon are visible)
• When the search input is clicked, the list of checkboxes becomes visible. Subsequent clicks don't hide the checkboxes.
• Each namespace is displayed with a checkbox, the icons that are currently visible are removed.
• Clicking on a line of an unselected namespace makes it selected. The checkbox is checked and background is highlighted.
• Clicking on a line of a selected namespace makes it unselected. The checkbox is unchecked and background is not highlighted.
• Namespaces and their talk namespaces are bundled together visually by a thick gray border (see Mock or Recent Changes).
• Typing in the search input field shows only those checkboxes where the label matches the typed text.
• Clicking on any part of the page outside the widget hides the checkboxes.

Background: see T202370