Add additional checks on incoming MLS messages

[stefanwire]

https://wearezeta.atlassian.net/browse/FS-797

Removed the test "user tries to remove themselves from conversation". In the test, the group creator was creating a remove commit which was then send by Bob to the backend. It is not working anymore, since now it is made sure by the backend that Bob creates all his commits himself. Switching to Bob creating the remove commit for his clients won't work either, since OpenMLS would forbid to create a commit in which the user even removes one of his/her own clients. Thus, the test was beyond repairable. However, there might be the option to craft a remove commit for Bob without OpenMLS later on when crafting MLS messages is available.

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• changelog.d contains the following bits of information (details):
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.

[pcapriotti]

Looks good. Minor comments below.