previously the back link went to choosing a library.
Now, if you view a district from a county, go back to the county page.
Otherwise, go back to the top level of the library.
Because the ‘All of’ choice appears above the search field, it shouldn’t
be filtered by the search field.
We can fix this by wrapping the sub-areas in a `<div>` and make the
selector more specific.
If a library has groups, we should show a link instead of selecting the
group directly.
Then we can give the user the choice of selecting the whole of that
group, or specific areas within the group.
For now the only libraries we have with groups are local authorities,
which group electoral wards.
