WebSubmit: doilookup function in webinterface

* Adds doilookup function to websubmit_webinterface that uses the
  crossrefutils and returns a dictionary with MARC codes as keys and
  values from crossref website as values (that can be used in
  submissions to prefill some fields based on the DOI).

Signed-off-by: Sebastian Witowski <sebastian.witowski@cern.ch>
The "ugly monster" has its reasons for existing; it has features that the "nice dictionary" does not have. (Moreover it has a whole set of API functions to help working with it.)  What about using more neutral language in code comments? :)