matrix-chat: reduce permission level for external users in public rooms

Recently there appeared a bit of spam via Matrix on stadtgestalten.org. The spam bot did not create a grouprise account, but just joined two (or more?) public rooms and emitted a message.

Other users in the room can delete the spam message, since everyone is a moderator.

But they cannot remove the spam users, since the spam user has moderation permissions, too.

I think, we should reduce the default permission level in public rooms to standard and instead raise the individual permission level of all automatically invited people to moderator (i.e. all members of the corresponding grouprise group).