Implement server-wide open group user bans and unbans.

[ianmacd]

This adds to Session the ability to ban a user not just from a single group, but from an entire SOGS.

To successfully ban (or unban) a user across a whole server, the executor must be a global moderator of the SOGS instance.

When banning a user, the global moderator may opt to also remove all of the user's messages from the server. This requires PySOGS > 0.3.7 to allow the simultaneous deletion of messages from multiple groups. See oxen-io/session-pysogs@2c8e4f1535bbd2cc676fa46914c691d2332cb41f.

This has been tested with Session 1.10.4 in combination with open.getsession.org and sog.caliban.org, both of which have been updated to support server-wide banning.

Contributor checklist:

• [x] My commits are in nice logical chunks with good commit messages
• [x] My changes are rebased on the latest clearnet branch
• [x] A yarn ready run passes successfully (more about tests here)
• [x] My changes are ready to be shipped to users