FEATURE: Experimental support for group membership via google auth (#14835)

This commit introduces a new site setting "google_oauth2_hd_groups". If enabled, group information will be fetched from Google during authentication, and stored in the Discourse database. These 'associated groups' can be connected to a Discourse group via the "Membership" tab of the group preferences UI. 

The majority of the implementation is generic, so we will be able to add support to more authentication methods in the near future.

https://meta.discourse.org/t/managing-group-membership-via-authentication/175950

app/models/group.rb

@@ -21,6 +21,7 @@ class Group < ActiveRecord::Base
   has_many :group_users, dependent: :destroy
   has_many :group_requests, dependent: :destroy
   has_many :group_mentions, dependent: :destroy
+  has_many :group_associated_groups, dependent: :destroy
 
   has_many :group_archived_messages, dependent: :destroy
 
@@ -32,6 +33,7 @@ class Group < ActiveRecord::Base
   has_many :reviewables, foreign_key: :reviewable_by_group_id, dependent: :nullify
   has_many :group_category_notification_defaults, dependent: :destroy
   has_many :group_tag_notification_defaults, dependent: :destroy
+  has_many :associated_groups, through: :group_associated_groups, dependent: :destroy
 
   belongs_to :flair_upload, class_name: 'Upload'
   belongs_to :smtp_updated_by, class_name: 'User'
@@ -768,6 +770,20 @@ class Group < ActiveRecord::Base
     self
   end
 
+  def add_automatically(user, subject: nil)
+    if users.exclude?(user) && add(user)
+      logger = GroupActionLogger.new(Discourse.system_user, self)
+      logger.log_add_user_to_group(user, subject)
+    end
+  end
+
+  def remove_automatically(user, subject: nil)
+    if users.include?(user) && remove(user)
+      logger = GroupActionLogger.new(Discourse.system_user, self)
+      logger.log_remove_user_from_group(user, subject)
+    end
+  end
+
   def staff?
     STAFF_GROUPS.include?(self.name.to_sym)
   end