Added Access control for admins / course instructors

[M97Chahboun]

fix #19

• Fix create new user issue

[M97Chahboun]

@alexeygrigorev Please check this PR and let me know if you have any notes/issues

[M97Chahboun]

@alexeygrigorev quick reminder.

[alexeygrigorev]

Apologies for the delay

I'm trying to run it and this is what happens:

Traceback (most recent call last):
  File "C:\Users\alexe\Anaconda3\lib\threading.py", line 980, in _bootstrap_inner
    self.run()
  File "C:\Users\alexe\Anaconda3\lib\threading.py", line 917, in run
    self._target(*self._args, **self._kwargs)
  File "C:\Users\alexe\.virtualenvs\course-management-platform-wiAsnpQu\lib\site-packages\django\utils\autoreload.py", line 64, in wrapper
    fn(*args, **kwargs)
  File "C:\Users\alexe\.virtualenvs\course-management-platform-wiAsnpQu\lib\site-packages\django\core\management\commands\runserver.py", line 133, in inner_run
    self.check(display_num_errors=True)
  File "C:\Users\alexe\.virtualenvs\course-management-platform-wiAsnpQu\lib\site-packages\django\core\management\base.py", line 556, in check
    raise SystemCheckError(msg)
django.core.management.base.SystemCheckError: SystemCheckError: System check identified some issues:

ERRORS:
<function ManyToManyField.contribute_to_class.<locals>.resolve_through_model at 0x000001AAA2F65EE0>: (models.E022) <function ManyToManyField.contribute_to_class.<locals>.resolve_through_model at 0x000001AAA2F65EE0> contains a lazy reference to courses.instructor, but app 'courses' doesn't provide model 'instructor'.
courses.Course.instructor: (fields.E331) Field specifies a many-to-many relation through model 'Instructor', which has not been installed.

System check identified 2 issues (0 silenced).

Have you encountered it?

[M97Chahboun]

@alexeygrigorev NP I will check it today