improve and clarify party avatar sort options for header - fixes https://github.com/HabitRPG/habitrpg/issues/2239

Requires https://github.com/HabitRPG/habitrpg-shared/pull/309

- add sort by avatar name (sort order is Z -> A due to how the original code is written [there are good reasons]; can be fixed later if needed)
- add sort by background (avatars with no backgrounds appear at end)
- clarify wording for sort option to indicate that it's only for the avatars in the header, not for the list of party member names
- remove sort feature from guilds pages because it has nothing to do with guilds