bibdocfile: improve file format policies tuning
bibdocfile: improve file format policies tuning

* Moves CFG_WEBSUBMIT_DESIRED_CONVERSIONS into invenio.conf
  so that users can tune desired automatic file conversions.

* Introduces CFG_BIBDOCFILE_BEST_FORMATS_TO_EXTRACT_TEXT_FROM
  into invenio.conf so that users can tune the choice of
  which file format should be preferred for text extraction.

* Introduces CFG_BIBFORMAT_HIDDEN_FILE_FORMATS into invenio.conf
  so that users can choose which formats should be in general
  not advertised.

inveniosoftware/invenio-formatter,inveniosoftware/invenio-formatter,inveniosoftware/invenio-formatter,tiborsimko/invenio-formatter,tiborsimko/invenio-formatter,tiborsimko/invenio-formatter