Feature Request: Categorise by semester

[chew01]

There are a lot of groups on the TeleNUS directory that don't have consistent names or formats.

*The effect was more pronounced prior to March 2024 when some of the groups went missing for some reason...)

This is fine, but it can be improved by allowing group admins to pick which semester their group belongs to, or none, so we don't have to rely on explicit group naming.