Cloned experiment or winner mailing should have type standalone

The mailings of an A/B test have a `mailing_type` which can be experiment "experiment" or "winner". Other mailings have the type "standalone".

When a user re-use a mailing that was part of an A/B test, the new mailing is a standalone one but has the type of the cloned mailing in database. Any code that reads this type will be misled and may behave wrongly. As a result the type value is not reliable, an additional check that the mailing is present in `civicrm_mailing_abtest` has to be done.

The mailing type of a cloned mailing should be always be forced to "standalone".