[REF] mail: mail_channel update

Some features from live_chat have been moved to the mail module :
- channels now have members, replacing followers;
- a decorated m2m is used to link channels and members. It stores the last
seen message on the channel for a specific partner;
- channels do not create menu entries anymore, because the
NewChatter will have its own display and use of channels;
- access rights have been updated accordingly, using members instead of
followers