DEV: Add delegated Group#human_users scope (#25398)

Some preparatory refactoring as we're working on TL groups for the system user. On User we have a scope #human_users to exclude the system user, DiscoBot, etc. This PR adds the same scope (delegated to User) on Group.

app/models/group.rb

@@ -24,6 +24,7 @@ class Group < ActiveRecord::Base
 
   has_many :categories, through: :category_groups
   has_many :users, through: :group_users
+  has_many :human_users, -> { human_users }, through: :group_users, source: :user
   has_many :requesters, through: :group_requests, source: :user
   has_many :group_histories, dependent: :destroy
   has_many :category_reviews,