membership cache

[kpsherva]

• check if cache is cleared on logout, if not hook into the logout function to clear the cache (useful when the user loses externally managed group membership)
• make sure the group cannot be removed from the members list if they are the last owner
• what if externally managed group is deleted while it is the only owner? - maybe we need to require non-exterally managed group or user to be owner of a commmunity to avoid this case
• what happens if externally managed group is deleted upstream? does it disappear from the membership list (probably yes, like user)