[api] rework how subscriptions work …

We need to be able to have multiple roles per event - and let users override
them. The previous model only allowed on role per event, which might have been
'none'

But if users want to still get mails for 'creator' role, but ignore 'maintainer',
they need to be able to black out 'maintainer' without touching 'creator' role
Merge branch 'rework_subscriptions' into notification_ui

Conflicts:
	src/api/app/models/event/request.rb
	src/api/app/models/event_find_subscribers.rb
	src/api/test/fixtures/event_subscriptions.yml