Adds staff and instructor users of the master course to CCX

[amir-qayyum-khan]

Background

fixes mitocw#126, fixes mitocw#155, fixes mitocw#115 and fixes mitocw#205

This is an updated version of https://github.com/edx/edx-platform/pull/10877 which had to be reverted.

What is done in this PR

Studio Updates:

• Remove ccx from staff dashboard inside studio. Because a master course can have unlimited ccx, staff's dashboard can be populated with a huge list of ccx along with master course

LMS Updates:

• This is for course staff to see what ccx coaches are doing with course.
• Now when a coach creates a new ccx, I am assigning staff role on this ccx course to all staff of master course.
• Added instructors of master course as instructor(s) in ccx.
• Also adds the master course staff users to new CCX created via the CCX REST APIs
• Note: This code will add/enrol staff and instructors in ccx and now max student limit for ccx depends on number of staff and admins.
• Fixed view as student issue on ccx, now staff can view as student and see how ccx view will appear to student
• Fixed display name on ccx coach dashboard. Previously it was show display name of master course on coach dashboard.

@pdpinch @giocalitri

• Staff on Master course
  
• Staff on CCX
  
• Admin/instructors on master course
  
• Admin/instructors on ccx
  

Fixed view as student

Fixed display name of ccx on coach dashboard.

[pdpinch]

I updated the cover letter a bit.