Avoid MLS messages in sender client's event stream

[stefanwire]

Checklist

• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines

[pcapriotti]

Looks good. Although maybe it would be good to have a test that the sender client is indeed not receiving these messages. And maybe some comments for our future selves explaining that this behaviour is indeed pretty strange, but it's what the clients want.