Improve filename building for Group DMs

[flauschzelle]

If a DM group has no official name contained in the archive data, the filename is constructed from the handles of the most active users in the group. In groups with mostly inactive users (i.e. users who have sent 0 messages to the group in the archived time interval), this did not work correctly before, because inactive users were not counted for the name construction.
If the handles of the more active users could not be found, then the filename would just have been made from the group's id instead of including handles of inactive users.

For more human-readable filenames, IDs are not useful. So now the inactive users' handles are used as a fallback if there are not enough known handles of active users.