fix bug 1087527 - use the SessionAwareClient in more tests

This seems to have been a leftover from 5743ed2059ccd4dc02ed7ccb789d2f637c2dfc2f.
fix bug 1072380 - make sure we raise validation error at the right spot of the call chain.

The problem was that the validation error was raised at a stage where it didn't trigger a re-rendering of the page.
This also contains some refactoring of the newsletter feature to encapsulate it in the NewsletterForm.
Merge pull request #2845 from jezdez/bug1072380

fix bug 1072380 - make sure we raise validation error at the right spot of the call chain