Admission-based registrations mode (#10250)

Fix #6856
Fix #6951

db/migrate/20190307234537_add_approved_to_users.rb

@@ -0,0 +1,23 @@
+require Rails.root.join('lib', 'mastodon', 'migration_helpers')
+
+class AddApprovedToUsers < ActiveRecord::Migration[5.2]
+  include Mastodon::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  def up
+    safety_assured do
+      add_column_with_default(
+        :users,
+        :approved,
+        :bool,
+        allow_null: false,
+        default: true
+      )
+    end
+  end
+
+  def down
+    remove_column :users, :approved
+  end
+end