ext: arXiv Flask extension database search fix

* Adds an instance wide search parameter prefix to perform the database
  search.

* NOTE: for the search to be performed in the database, the arXiv field
  prefix for the corresponding instance config file should be added to
  invenio.cfg as ARXIV_SEARCH_PREFIX
  (e.g. ARXIV_SEARCH_PREFIX = u'035__a:oai:arXiv.org:').
`archiv = self.app.config.get("ARXIV_SEARCH_PREFIX", "") + (arxiv or request.args.get("arxiv"))`