This commit adds users to the appropriate system user group
based on their role. We also change the user groups when
changing role of the user.
We also add migration to add existing users to the appropriate
user groups.
This commit adds update_users_in_full_members_system_group which
is currently used to update the full members group on changing
role of a user. This function will be modified in next commit such
that it can be used to update full members group on changing
waiting_period_threshold setting of realm.
Sahil Batra