Ability of moderators to remove (& possibly block) members

[SK53]

Example use case: person violates group Code of Conduct in a f2f activity, but is still active on OSM. One may not wish that they continue as a member of the group or get messaged about group activities. Unless they are also banned or blocked on OSM they will still have the same rights as any other member of a microcosm.

At present one can just drop them from a group mailing list. Obviously they can still see information about the event (as now). At least one complication is that they may be made aware when removed from a given group. Another one is if they persist in rejoining.

Probably fine for a later release, but it is probably necessary.