Implement blocking command (XEP-0191)
https://xmpp.org/extensions/xep-0191.html allows to block a user from client, and it is a feature requested for advanced client in 2018 compliance suite (https://xmpp.org/extensions/xep-0387.html).
The idea here is to implement it first with text commands, so it would be usable in all frontends without changing the UI.
Something like
/block [-u] [ROOM_NICK|JID]
/block_list
where "-u" is for unblocking.
Check https://wiki.goffi.org/wiki/Coding_style/en#Text_commands for details on text commands docstrings.
That would be good as a first implementation.
In a second time we can add jp commands, or menu in frontends, need to be discussed.