Read in new 'backends' config item and use it to display a dropdown in the
navbar. Also, use the new config item to connect to the api.
Properly read in other backend items from the config, and react to changes
by reloading collections data (fetch with backend_names param).
Allow for empty 'backends' config. If there is exactly one backend (global
or otherwise), don't display the dropdown.
Allow empty/missing 'backgrounds' objects.