Currently themes machinery works in such way that if we rely on
default theme in our branded theme, we have to include them both into
AVAILABLE_THEMES setting, otherwise default theme assets will be
unavailable and the branded theme assets compilation will fail. On
the other hand, mentioning them both leads to theme picker being
shown - which we would like to avoid (per marketing wish).
SELECTABLE_THEMES setting was added to allow limiting the user facing
themes by configuration.
Closes-bug: #1564543
Co-Authored-By: Ivan Kolodyazhny <e0ne@e0ne.info>
Change-Id: Ic00a9201d2d352685b1089a37a25987b75d6636d
958e522d40