Add option to make a full backup of an account

The daemon currently supports exporting an "archive" of an account and importing it on a different device. Such an archive contains, among other things, metadata about the account's conversations. However, it does not contain the actual content of these conversations (i.e. the messages); this information is downloaded from the first device (or from the contacts) after the archive has been imported.

It would be good to also add the option of exporting (and importing) a full backup of an account. This would make it easier for some users to add their account to a new device (e.g. if their Internet connection is slow, or if the new device can't connect to the old one for some reason). It would also reduce the risk of data loss in some cases (for example, if a user has a conversation with themselves and their account is only present on one device, then they won't be able to restore that conversation from an archive if the account's files are deleted from that device).

added sprintbacklog typefeature request labels

1. This can help if end-users have a internet plan with a limited number of gigabytes per month.
2. This can save end-users money if they are charged money per megabyte of Internet usage.

Thank you

assigned to @ffauteux-chapleau