Correctly form slugs for files with spaces in their names
Adapts document slugs to the way gridfs stores filenames
Merge pull request #41 from flavioamieiro/bugfix/document-slug-generation

Bugfix/document slug generation
Fixes KeyError when document language was an empty string

This happened when the document mimetype was not recognized by the extractor
worker in the backend.
Showns 'Unknown' if the document language wasn't detected
Adds translation to 'Unknown' (language)
Merge branch 'bugfix/empty_language_on_document_page' into develop