Fix notifications

https://gitlab.com/soapbox-pub/soapbox/merge_requests/120/diffs#diff-content-917c7fb80cf426cfae1e7baaa21b3d90ccb2b29c

db/migrate/20191201204843_add_last_read_notification_to_user.rb

@@ -0,0 +1,5 @@
+class AddLastReadNotificationToUser < ActiveRecord::Migration[5.2]
+  def change
+    add_column :users, :last_read_notification, :bigint
+  end
+end

db/migrate/20191201205006_add_foreign_key_constraint_to_user_last_read_notification.rb

@@ -0,0 +1,5 @@
+class AddForeignKeyConstraintToUserLastReadNotification < ActiveRecord::Migration[5.2]
+  def change
+    add_foreign_key :users, :notifications, column: :last_read_notification, on_delete: :nullify, validate: false
+  end
+end

db/migrate/20191201210057_validate_add_foreign_key_constraint_to_user_last_read_notification.rb

@@ -0,0 +1,5 @@
+class ValidateAddForeignKeyConstraintToUserLastReadNotification < ActiveRecord::Migration[5.2]
+  def change
+    validate_foreign_key :users, :notifications
+  end
+end