Notifications refactoring

[capital-G]

Started some work on #565 which turned into its own PR (and is therefore based on that branch). Can be merged after #565 has been merged.

Simply a refactoring of the "who needs to tell who something was changed" for the graphql subscriptions, makes the code a a bit nicer.

Hopefully doesn't break something, but all tests are running.

[github-actions]

File	Coverage
All files	82%	✅
gencaster/db_logging.py	96%	✅
gencaster/distributor.py	68%	✅
gencaster/schema.py	55%	✅
gencaster/urls.py	76%	✅
osc_server/models.py	91%	✅
osc_server/server.py	55%	✅
story_graph/engine.py	91%	✅
story_graph/markdown_parser.py	91%	✅
story_graph/models.py	97%	✅
story_graph/types.py	93%	✅
stream/frontend_types.py	94%	✅
stream/models.py	92%	✅
stream/types.py	99%	✅

Minimum allowed coverage is 50%

Generated by 🐒 cobertura-action against bcb2443

[vin-ni]

approved