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

db/migrate/20211106085344_create_associated_groups.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+class CreateAssociatedGroups < ActiveRecord::Migration[6.1]
+  def change
+    create_table :associated_groups do |t|
+      t.string :name, null: false
+      t.string :provider_name, null: false
+      t.string :provider_id, null: false
+      t.datetime :last_used, null: false, default: -> { "CURRENT_TIMESTAMP" }
+
+      t.timestamps
+    end
+
+    add_index :associated_groups, %i[provider_name provider_id], unique: true, name: 'associated_groups_provider_id'
+  end
+end

db/migrate/20211106085527_create_user_associated_groups.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+class CreateUserAssociatedGroups < ActiveRecord::Migration[6.1]
+  def change
+    create_table :user_associated_groups do |t|
+      t.references :user, null: false
+      t.references :associated_group, null: false
+
+      t.timestamps
+    end
+
+    add_index :user_associated_groups, %i[user_id associated_group_id], unique: true, name: 'index_user_associated_groups'
+  end
+end

db/migrate/20211106085605_create_group_associated_groups.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+class CreateGroupAssociatedGroups < ActiveRecord::Migration[6.1]
+  def change
+    create_table :group_associated_groups do |t|
+      t.references :group, null: false
+      t.references :associated_group, null: false
+
+      t.timestamps
+    end
+
+    add_index :group_associated_groups, %i[group_id associated_group_id], unique: true, name: 'index_group_associated_groups'
+  end
+end