add group management features and slightly improve user management code

[SaphiraKai]

I wrote this at like 5AM my time so uh probably wise to check this carefully before merging lmao.

I have tested it and it seems to work fine, but it's quite late so I may have missed something, I'll look at it again in the morning. Er... Later in the morning.

Anyway, this PR adds group management features to the Users section. Specifically, it adds:

• Adding groups
• Removing groups
• Adding a user to a group
• Removing a user from a group

Also made some slight changes to my existing user management code, including a proper error message when attempting to create a user that already exists.