Implement equal fields constraint in schema

[luisa-beerboom]

See OpenSlides/openslides-backend#3379

TODOs aside from The Obvious™

• Look into constraint triggers
• What about constant models.yml field def? Doesn't seem to be handled in any way?
• generate_sql_schema fails on option/content_object_id, bc it tries to get meeting_id of user
• Changed some stuff about the schema generation that may impact generated field names. perhaps ask @peb-adr how to potentially deal with that or change it back -> Looked at it: Apparently it was an edge case that wasn't present in our actual code to begin with.
• CI fails
• refactor and remove the debugging commands (all those print statements, etc)
• cleanup

NOTES:

• Possibly the option/content_object_id meeting_id-equal_fields might have not been previously checked for users

[vkrasnovyd]

Resolves #294

[vkrasnovyd]

Checked only the updated trigger names generation (the change that I've requested). That's exactly what I wanted to see.

[hjanott]

Thanks for the hard work. Looks ready for me now.