This prep commit adds a new OnboardingUserMessage model
that will be used to mark the new onboarding messages
for new users as unread and the first message of each
onboarding topic as starred.
This table won't include the old onboarding messages.
This commit updates code, majorly in tests, to use
setting values from enums instead of directly using
the constants defined in Realm.
We still have those constants defined Realm as they
are used in a couple of places where the same code
is used for different settings. These will be
handled later.
We now "first_message_id" of the stream on the deletion of the first
message that was sent to it. This results in 1 extra query when any
stream message is deleted and 3 extra queries when the first message
sent to any stream is deleted.
Fixes#28877.
Anders Kaseorg
Prakhar Pratyush
Sahil Batra
Vector73
Alex Vandiver