Make impersonate_user local to a course

[isuruf]

If a user is a super-user, they'll be able to impersonate anyone. If not, a user (ta) can impersonate a user who they have access to (student) in that particular course.

Note that when impersonating, a ta will only be able to access the courses of that student that he/she has permission for. Access to other courses will be denied. (Any urls starting with /course/)

One possible issue with this approach is that a ta will be able to see the home page of the student, which means they'll be able to see the titles of some private courses that the student has access to.

[inducer]

Thanks for your work on this!

[inducer]

Could you merge in master to get the latest CI? Also, could you remind me what the status is here?

[codecov[bot]]

Codecov Report

Merging #670 into master will decrease coverage by 32.76%. The diff coverage is 12.5%.

@@             Coverage Diff             @@
##           master     #670       +/-   ##
===========================================
- Coverage   96.59%   63.83%   -32.77%     
===========================================
  Files          45       45               
  Lines       11105    11126       +21     
  Branches     2065     2072        +7     
===========================================
- Hits        10727     7102     -3625     
- Misses        302     3414     +3112     
- Partials       76      610      +534

Impacted Files	Coverage Δ
relate/urls.py	90.9% <ø> (-9.1%)	:arrow_down:
course/auth.py	24.8% <12.5%> (-59.69%)	:arrow_down:
course/calendar.py	20.17% <0%> (-79.83%)	:arrow_down:
course/exam.py	23.94% <0%> (-76.06%)	:arrow_down:
course/enrollment.py	25.94% <0%> (-72.88%)	:arrow_down:
accounts/utils.py	37.6% <0%> (-60.69%)	:arrow_down:
course/views.py	41.82% <0%> (-58.18%)	:arrow_down:
relate/checks.py	42.28% <0%> (-57.72%)	:arrow_down:
course/validation.py	50.65% <0%> (-49.17%)	:arrow_down:
... and 21 more

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[isuruf]

Also, could you remind me what the status is here?

Need to write tests.