Federation: support read receipts

[pcapriotti]

The initial purpose of this PR was to add federation behaviour for receipt mode update, which it does. However, in an effort to avoid copy-pasting code, it also unifies all the federation-aware conversation update code paths, and as a consequence the diff ended up being quite large.

The core of the unified interface is the updateLocalConversation function, which performs an update and notifies local and remote users.

Note: the remove member update is a bit special, because it was implemented using checked exceptions. To make it compatible with the other updates, checked exceptions are not currently used, although they are still present in some type signatures.

To make the unification possible, a number of other changes were necessary:

• Removed checked add member wrapper. This wasn't either safe (because it didn't encompass all the necessary size checks) nor necessary (because the checks were already performed by ensureMemberLimit).
• Aligned conversation update RPC to endpoint interface. The generalised RPC interface is not necessary for now (it can always be regeneralised later if needed), and making it the same as the public interface simplifies some things.
• Separated self-member updates from other-member updates. These are really different anyway, and there was nothing gained by having a common interface.
• Added some general code to deal with local and remote members uniformly. This simplifies many functions that can operate on both local and remote members. For example member removal, which is also used for leaving a remote conversation.
• Introduced a UserList type, representing a list of users partitioned into locals and remotes.
• Removed many of the convenience functions to add members to a conversation. The default admin role functionality is now implemented using a ToUserRole type class.
• Event creation for adding and removing members or creating conversations is now done outside the Data module. This will make it possible to unify the event creation and propagation logic across many conversation endpoints.
• Many functions operating on lists have been generalised to Foldable, so they can work uniformly with UserLists and normal lists.
• The ConvSizeChecked newtype was broken, because the Functor instance would allow to break the invariant encoded by the type. This is now fixed, and the type can now wrap an arbitrary Foldable.
• Support for creating managed conversations has been dropped

This is part of https://wearezeta.atlassian.net/browse/SQCORE-885 (federated conversation metadata updates).

Checklist

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[fisx]

and as a consequence the diff ended up being quite large.

My first thought was "that should have been split up, then!". My second, 200ms later: "this keeps happening to me, too, why is this so hard?!" :)

[pcapriotti]

and as a consequence the diff ended up being quite large.

My first thought was "that should have been split up, then!". My second, 200ms later: "this keeps happening to me, too, why is this so hard?!" :)

It requires a lot of discipline to keep changes separated when refactoring things on a larger scale, and I don't always have it. It wouldn't be impossible to carefully split the changes into meaningful commits after the fact, but it would take considerable time, and I thought it might be better spent for other things.