Fixes inconsistent timezone in emails and lesson time edit, fixes #34

Some best practices may fix that famous issue #34, I hope. All the tests are working and apart from the different Time.zone in config/environment/test.rb, I really don't see how to make the tests better, I ensured myself that the tests cover sending notification mails with two different school timezones and it does!
I hope it'll work, else I really don't understand what's happening for now; we'll unfortunately really see if it works with the next real notification mail sent.