Extend role assignment authorization

imported from https://todo.sr.ht/~luma_inhibitor/cables/64

When the .role command is used to manipulate another user's roles, impose additional checks on whether or not this is an allowed action before carrying it out.

1. Check that the user is assigned a role from a list of moderator roles
2. Check if the issuing user's highest role is above the role that they are attempting to change assignment of