* Introduce ActiveMLSConversationData

This changes the conversation and subconversation metadata so that
epoch, epoch timestamp and ciphersuites are all simultaneously optional.
This makes it possible not to set a ciphersuite for conversations until
they receive a commit.

* Fix assertions in integration tests

* Add more versioned conversation endpoints

* Make SubConversation record versioned

* Adapt galley to versioning changes

* Adapt and expand conversation golden tests

* Fix arbitrary instance of ConversationMLSData

* Test old versions of conversation metadata

* Restore epoch field in Conversation serialisation

* Add CHANGELOG entry

* Lint

* Test MLS 1-1 with other ciphersuites

* Fix assertion in galley integration test

* fixup! Fix assertion in galley integration test