Fix old migrations failing because of strong_migrations update (#12787)

Fixes #12768

Some migrations were overlooked in #12692

db/migrate/20180206000000_change_user_id_nonnullable.rb

@@ -1,6 +1,8 @@
 class ChangeUserIdNonnullable < ActiveRecord::Migration[5.1]
   def change
+    safety_assured do
       change_column_null :invites, :user_id, false
       change_column_null :web_settings, :user_id, false
     end
   end
+end

db/migrate/20191031163205_change_list_account_follow_nullable.rb

@@ -1,5 +1,7 @@
 class ChangeListAccountFollowNullable < ActiveRecord::Migration[5.1]
   def change
+    safety_assured do
       change_column_null :list_accounts, :follow_id, true
     end
   end
+end