[rel-DB] Complete checker tests

[vkrasnovyd]

Changes to presenters and tests:

• As discussed with Raimund, preserved the logic of the presenters and deleted only the tests with inconsistent data. The remaining tests make sure that the checker does not scream when data is correct;
• Removed fields that are not in export_helper.get_fields_for_export in the check_database_all (logic similar to export_meeting). Reason: checker was screaming because of the calculated fields pulled from db (like mediafile.owner_id_organization_id or projection.content_object_id_meeting_id).

Changes to checker:

• Added more precise field type validation for GenericRelationField and GenericRelationListField to get errors about incorrect field value type;
• Remove TODO-comment without actually making any changes. reason: the 2 areas are making slightly different checks so I'd better leave optimizing them for rel-DB 2;
• Added some missing points, fixed number of whitespaces for better readability;
• Merged "state_extension" and "recommendation_extension" validation in check_special_fields(). Reasons:
  • Logic is completely the same.
  • Error message for recommendation_extension had duplications. Example:

motion/1/recommendation_extension: Relation Error: Found motion/3 in recommendation_extension but not in models.
motion/1/recommendation_extension: Relation Error: Found theme/1 but only motion is allowed.
motion/1/recommendation_extension: Relation Error: Found motion/3 in recommendation_extension but not in models.
motion/1/recommendation_extension: Relation Error: Found theme/1 but only motion is allowed.

[hjanott]

I see many beautiful changes.

[hjanott]

I just found one line that should be removed before merging.

[vkrasnovyd]

@hjanott added some missing test cases, did a bit of cleanup in the checker itself and added a couple of todos for rel-db-2. Also re-enabled migration index check in the checker.