Teacher permissions update

[solumath]

When there's permission update for any of the public channels with new permissions for teachers, it is hard to recognize new changes. Maybe it would be better to show diff or something of that sort.
You can find the logger here. https://github.com/vutfitdiscord/rubbergod/blob/main/cogs/fitwide/cog.py#L624

[1ukastesar]

difflib.unified_diff() could be used for this purpose.