Registration emails 2

Contains:
* Autocomplete registration emailaddresses from users and accounts
* Prefill sentRegistrationEmailAddresses on sql-import
* Several fixes to registration email rendering
* Logo + agelimit + warning icons as images to registration email
* Fix dialogs in UI: Use flex for vertical centering; unify look&feel
* ClassificationForm: prefill billing when it's empty and the buyer is set.

Enjoy,