Make default backend more obvious and reduce visual clutter

Right now the default backend on the Settings page is not as obvious as
it could be.

This commit makes it more obvious by:
- Using bold text for the default backend's header, as we commonly do
  elsewhere
- Changing the text "(Default)" to "Default source"
- Moving the text over to the right where it replaces the "Make Default"
  action, which now hides for the backend that is already default

Overall this should make the default backend clearer and reduce visual
clutter.

BUG: 447009
FIXED-IN: 5.24