Rationalise trailing slashes from admin_base_url

Remove from admin_base_url (which wraps config vars like
{ADMIN_,}BASE_URL) and add to places in code which aren't
expecting it.
Test for admin_base_url in RS::Problem

Test that the slash is added in correct place (the base URL should not
have its own trailing slash).
[Zurich] stats export include 'hidden'

See https://github.com/mysociety/FixMyStreet-Commercial/issues/461

Again, we just need to substitute the %params line (which elides
hidden reports) for the less restrictive %date_params.

This commit includes very basic tests for the previously untested
?export=1 functionality.
Merge branch 'issues/commercial/460-trailing-slashes-on-base-url'
Merge branch 'issues/commercial/461-zurich-export-hidden-stats'