Change force_ssl settings and add protocol to mailer

SSL has been forced from within the engine until now. This moves that responsibility to the wrapper and can later be removed from the engine.

Also all links in e-mails are now https when ssl is enforced.
I think this is fine, but would be slightly better if it defaulted to on like @threedaymonk recommended when we did secure cookies in SCS. Something like this:

```ruby
ENV['SSL_ON'] != 'false'
```

That way if for whatever reason someone forgets to configure it now or in the future, it will "fail safe" i.e. be switched to the safe option, TLS on.