BibRank: increase rnkDOWNLOADS.file_format size

* Increses the size of the file_format column in rnkDOWNLOADS
  since this might contain both the format and subformat information.

Co-authored-by: Tibor Simko <tibor.simko@cern.ch>
installation: test presence of flask.ext.admin

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>
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