Use attributes.pot from musicbrainz server to generate up-to-date cover art types and medium formats lists 

This is the same principle we used for countries:
* retrieve and parse a pot file
* generate a python file from it
* use corresponding .po files to ensure translations, reducing translations redundancy between mb server and picard

The thing is a bit hacky here, because python gettext currently fails to handle msgctxt which are used in attributes.pot

To test:
```bash
$> python setup.py update_constants
$> python setup.py get_po_files
$> python setup.py build_ui -i
$> python setup.py build_locales -i
$> python tagger.py -d
```

`update_constants` will regenerate countries.py and attributes.py for server pot files.