deposit: import priorities

* Allows importing both from arxiv and from doi fields, regarding the proper
  priorities (first arxiv, then doi).

* Strips 'arxiv:' and 'doi:' prefixes from the identifier's values.

* Changes replacing flash messages to appending them to existing ones everytime
  there's a new one.

* Removes label from the import data submit button.

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>