ziafazal/api-fix-bug-progress-tab: fix progress

@mattdrayer @martynjames @dsego 
1) Fixed progress value by using total modules in the course as denominator. 
2) Changed completion avg to percentage. We have to change it on apros accordingly
3) Fixed bug in serialiser which was causing multiple calls to `auth_user` in CourseModuleCompletionList as reported by @chrisndodge  
It seems to me that total_completions is no longer the correct value to use on this line?  What you need is  total_actual_completions, yes?