Allow deletion of message history (for groups in particular)
Requires both a libCwtch change, and UX work to decide how to communicate this. We currently allow an all-or-nothing deletion with p2p, but do now have an equivilant for groups (other than purging the group key).
For both we should allow a clearing of the message cache, maybe "delete messages after X days" or "delete all messages after this message"
Deleting a branch is permanent. It CANNOT be undone. Continue?