Make migrations reversible

While I don't use this feature, there are developers who do. It's useful
when running migrations and changing branches.

I'm raising an `ActiveRecord::IrreversibleMigration` exception in every
`drop_table` migration because these migrations were all done before
version 1.0.0, and so making all of them reversible would be too much
work for little benefit.

db/migrate/20171115164152_add_time_zone_to_default_datetimes.rb

@@ -1,6 +1,11 @@
 class AddTimeZoneToDefaultDatetimes < ActiveRecord::Migration[4.2]
   def change
-    change_column_default :users, :password_changed_at, DateTime.new(2015, 1, 1, 1, 1, 1, "+00:00")
-    change_column_default :locks, :locked_until, DateTime.new(2000, 1, 1,  1, 1, 1, "+00:00")
+    change_column_default :users, :password_changed_at,
+      from: Time.zone.local(2015, 1, 1, 1, 1, 1),
+      to:   DateTime.new(2015, 1, 1, 1, 1, 1, "+00:00")
+
+    change_column_default :locks, :locked_until,
+      from: Time.zone.local(2000, 1, 1, 1, 1, 1),
+      to:   DateTime.new(2000, 1, 1, 1, 1, 1, "+00:00")
   end
 end