Add classification behavior to mails.

@lukasgraf 

- Add classification behavior to mail contenttype
- Init classification behavior fields with default values for existing mails.

Important: Check https://github.com/4teamwork/opengever.core/compare/feature_public_trial...mle-public_trial_classification?expand=1#diff-c8cc3da6244d95c930ecc3482d361ca0R31

I guess we need to fix this is ftw.builder, since the field has a default value (empty string), but if the mail is created with ftw.builder the stored default value is None, which is wrong.