Enrich musicians and bands items with membership statements

[MaxFrax]

Goal: to generate (musicians > band) and (band > musicians) statements

Expected statements:

• musician > band = Joey Ramone Q312387, member of P463, Ramones Q483407
• band > musician = Ramones Q483407, has part P527, Joey Ramone Q312387

Tasks:

• [ ] define the ORM entity for Discogs, see MusicBrainz;
• [ ] populate the artist > group relationship table for Discogs, see MusicBrainz here and here;
• [ ] implement the statement generator, get inspired by the validator