Display target identifier labels and descriptions in the MnM UI

[marfox]

• [ ] Lookup the labels for a given target ID;
• [ ] set the soweego.importer.models.mix_n_match.MnMEntry.ext_name to the target ID label in soweego.ingestor.mix_n_match_client._set_entry_fields;
• [ ] investigate how to add a description field, see #364 .

Current display:

Should become like:

N.B.: this story is also particularly important to avoid the creation of items like this: https://www.wikidata.org/w/index.php?title=Q70764379&oldid=1031413698 (spotted here: https://www.wikidata.org/wiki/Wikidata:Project_chat#soweego_1_review)

[marfox]

Raising priority as per #364