Fix broken links in breadcrumbs

This adds some extra checks to see if the breadcrumb url can be resolved, if it can, then we generate the breadcrumbs as normal. If we can't resolve the url it then we still display the breadcrumb, but we don't make it a link. This fixes pages like http://za-pombola.staging.mysociety.org/place/latlon/-33.925,18.424/ where the **latlon** breadcrumb previously linked to `/place/latlon`, which doesn't exist.

This also solves the problem of some of the breadcrumbs being relative rather than absolute links, which cause 404 errors.

Closes #801