Resolve "Die Medien an generierten Artikeln sollte an hand der Verteilung der Medien aus der echten Datenbasis erfolgen"

Closes #96 (closed)

• corrected mistake: assign_media_to_generated_articles now operates on generated articles, not cropped articles
• improved assignment of a medium when an article is generated: use medium distribution, not just a random medium

Closes #175 (closed)

• excludes wrongly_parsed 'pressespiegel' medium from being attached to generated articles

Closes #176 (closed)

• enforces uniqueness for some database fields like medium name, medium type name, user tag_id, ...
• fixes usage of Medium.objects.update_or_create in write_article_to_db that added the same medium multiple times