mmsd: Group MMS support

Although !768 (merged) has backend support for Group MMS, the functionality is broken. As shown in https://source.puri.sm/kop316/chatty/-/blob/master/src/users/chatty-mm-account.c#L316 , a Group MMS is not an IM and is CHATTY_PROTOCOL_MMS.

Group MMS is critical for basic MMS support in Chatty. What is needed to close the issue (there may be more? But testing is needed):

• Be able to add multiple contacts and create a new Group MMS. See !896 (merged)
• The "Chat Details" page of a Group MMS does not show the members of the chat. See !904 (merged)
• Be able to resolve contacts in individual chat members.
• When received, the "Title" of the Group MMS is just a comma seperated list of the phone numbers in the chat, and has no concept of contacts. There needs to be a way to change the title to reflect contact details.
• If Chatty is closed and reopened, a Group MMS can show up twice. This may be an error in !768 (merged) , but I am not sure.