Subscriptions manager have been a pain since the beginning, one of the problem is that thread and channels behave mostly the same but with various small difference which I expect to increase over time.
Trying to use subclasses for this case has proven to be a mistake, this commit now uses a class for each case (channel, thread) which for now contains a lot of duplication, which might be reduced in the future but has the merit to make reasoning about each case very simple.
This refactor is fixing a bug introduced in 90efdd7f9d which was causing the wrong channel to be unsubscribed, this shouldn't be possible anymore. We had tests for this which were disabled due to flakeyness, I will consider re-enabling them in the future.
Other notes:
- notices had been added to the subscriptions manager service, they have been moved into their own dedicated service: `ChatChannelNoticesManager`
- the `(each model)` trick used in `<ChatChannel />` since 90efdd7f9d to ensure atomicity has been applied to `<ChatThread />` too
dcaa719363