Improve the group messages interface: ## Story As a customer, I want to improve the group messages interface, so that members are more likely to participate in the conversation. ## Scope We should update the Group Messages interface to include the conversation inside of the group itself. The tab should be renamed to "Group Messages". In the tab, the sub-nav should be removed. The "Send Private Message" screen should be removed, in favor with a button which directs to the new message screen, with the group pre-selected. (Note: this requires "Create a new interface for sending new private messages" to be completed first). The conversation should appear, with members being able to see previous messages. When a member joins the group, immediately give them access to the group messages conversation; including in their private messages screen. When a member leaves the group, remove them from the group messages conversation. When a member joins or leave, show a notice in the conversation.