CRM-20577 Fix activities to record actual html if activity per contact

CRM-20577 Fix activities to record actual html if activity per contact

(At least, this covers contributions at this stage).

I have also tried to move towards clearer inputs & outpus on createActivities, with the
weirdness around contactIds being moved up to the forms themselves and
not being passed in. Would prefer not to see form passed in either but, next time.

Unit tests cover emails & activity create (for the first time)