refactor/rewrite, test coverage, merge of preview and form

Big piece, will need a review please (ping @Natim @thoas) :rose:

Summary:

* merge the preview functionality in the forms (use form's Meta.initial or get_value_for_param)
* dropped the iframe (which was a PITA regarding resizing and visualisation)
* 100% test coverage
* rewrote nearly all the tests so they are only testing units: this came with a huge gain in execution time. Tests now run around 10 times faster, even though there's more than three times as many tests as before
* improved the docs
* no more meta programming: dropped the MailForm's mail_class attribute, which is now passed to the constructor instead, which is easier to read, understand and maintain
* some cleanup, dropping of code no longer used, some refactor/rewrite