Hygiene: Introduce generic switcher

Duplicate template code = bad.

Also:
* Introduce a selected color. After talking to Kaity, we identified that
the visual editor toggle switcher was incorrectly using the progressive color
progressive != selected. Introduced a new color that Kaity had discussed with
Jared. Remove change of icons when selected.

Note: We add template partials to both VE Overlay and wikitext overlay
as it seems a class cannot inherit partials from multiple parents. Although
the partials of EditorOverlayBase would get inherited by EditorOverlay, the close
button in Overlay does not get inherited.

Change-Id: Id666a29219f76f9dce766ec746b9699b027389ca