The page_header macro includes an optional back link. Since the
page_header is always used inside `<main>`, where the back link should
not be, this stops setting the back link in the page header and instead
sets it in the new `backLink` block.
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.
