Multi-level region structure for sources web page

[vkrause]

https://transitous.org/sources/ groups sources by regions, but has both CA and US states on the top-level next to countries (including CA and US themselves), which makes this a bit confusing to navigate as it's not always obvious whether a source is on the country or state level.

A possible improvement would be to have countries at the top level and have their states below on a second level. That still leaves one outlier though, the EU category.

[Altonss]

See issue #756 :)