connector: InvenioConnector URL validation

* InvenioConnector will now throw an InvenioConnectorServerError
  exception is the given URL is invalid.

Signed-off-by: Graham R. Armstrong <graham.richard.armstrong@cern.ch>
utils: slugify text function addition

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Collection Templates

* Introduce collection specific templates collection_{name}.html with
  fallback to the default collection.html