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  
Can we rename this variable to "total_possible_completions", which makes more sense and will help to clear up the confusion I noted on line 1624