Generate XDF from database state

Related to #726

Database schema

• Safe relation (self-references) for several resources:
  • dokumensteckbrief_relation
  • schema_relation
  • datenfeldgruppe_relation
  • datenfeld_relation
• Add several legacy-columns with mutable data, that is only relevant for xdf2-Regeln

database.py

• Combine all necessary operations regarding relations and references to Regeln for Datenschemata, Gruppen, Felder and Regeln into one create_XXXXXX and one update_XXXXXX command respetively for each resource.
• Update the save_steckbrief command to also handle all necessary updates to relations
• Remove unused old queries that where only necessary before when creating a new Datenschema, Gruppe or Feld.

Tests

• Create one simple test for xdf2 and xdf3 respectively to validate the xdf export:
  • (1) Create a full schema message with all possible values filled
  • (2) Save to the database
  • (3) Reconstruct the message from the database, and assert that the result is identical to the source