Handle service urls set to 'X' in local_interaction importer

The sourvce data has some entries set to a url of 'X'  Previously, this
was being imported as a URL, and surfaced on the site.  This changes the
behaviour to treat an 'X' as a flag to indicate that a URL doesn't exist
for that service.
Merge pull request #114 from alphagov/local_interactions_import_handle_x_urls

Handle service urls set to 'X' in local_interaction importer