Added an admin setting which allows the administrator to set the available layouts for users and which
defaults to all layouts being available, or the card layout only if no layouts are enabled.
If only one layout is enabled, the dropdown `nav-display-selector` template will not be displayed as
it can no longer be utilised.
If the user preference is set to a layout that is no longer available, this is ignored and the first
available layout defaulted to.
Added an administrator setting to set the visibility of the course category for the my overview
block on the users dashboard to support usage scenarios where displaying the course categories will
clutter course overviews.
