BibDocFile: additional mimetypes support

* Introduces new CFG_BIBDOCFILE_ADDITIONAL_KNOWN_MIMETYPES config
  variable to be able to recognize new mimetypes from a given file
  extension (useful in case an old mimetypes Python module is used).

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>
WebSearch: unit test disabling for CFG_CERN_SITE

* Disables non-applicable unit test (because of different
  index setup) when running on CFG_CERN_SITE.

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>
Merge branch 'maint-1.1'

* maint-1.1:
  WebSearch: unit test disabling for CFG_CERN_SITE
Merge branch 'master' into next

* master:
  WebSearch: unit test disabling for CFG_CERN_SITE
  BibDocFile: additional mimetypes support

* Merge note: amended for lazy imports.

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

Conflicts:
	modules/websearch/lib/search_engine_unit_tests.py
pidstore: initial release

* Implementation of persistent identifier store for managing
  registration of persistent identifiers for Invenio objects.

Co-authored-by: Esteban J. G. Gabancho <esteban.gabancho@cern.ch>