[MCKIN-8417] Optimize OrganizationsViewSet.users API

[Agrendalath]

This PR optimizes listing and removing users in organizations.

Testing instructions (for solutions devstack):

Add a test organization and some users:

• add a test organization:

curl -X POST \
  http://localhost:8000/api/server/organizations/ \
  -H 'Cache-Control: no-cache' \
  -H 'Content-Type: application/json' \
  -H 'Postman-Token: b38bc64c-f147-493e-a0e1-8fd6df8b147b' \
  -H 'X-Edx-Api-Key: PUT_YOUR_API_KEY_HERE' \
  -H 'content-type: multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW' \
  -F name=TestOrganization

• add some users to the organization (the best would be to have more than 1 user there):

curl -X POST \
  http://localhost:8000/api/server/organizations/1/users/ \
  -H 'Cache-Control: no-cache' \
  -H 'Content-Type: application/json' \
  -H 'Postman-Token: 6ca75b56-1521-442c-80fa-382f313c6f95' \
  -H 'X-Edx-Api-Key: PUT_YOUR_API_KEY_HERE' \
  -H 'content-type: multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW' \
  -F id=5

(change id of user in the last line)

Install silk:

• run pip install django-silk==2.0.0 (3.0.0 dropped Django<1.11.0 support),
• add the following to common.py:

MIDDLEWARE = [
    ...
    'silk.middleware.SilkyMiddleware',
    ...
]

INSTALLED_APPS = (
    ...
    'silk'
)

• add urlpatterns += [url(r'^silk/', include('silk.urls', namespace='silk'))] to urls.py,
• run ./manage.py lms migrate silk --settings=devstack,

Make requests:

• make a couple (single requests are pretty inaccurate) of GET and DELETE requests on the edx-solutions:master branch
  GET (adjust course_id in URL param):

curl -X GET \
  'http://localhost:8000/api/server/organizations/1/users/?include_course_counts=true&include_grades=true&course_id=123/123/2018_T4' \
  -H 'Cache-Control: no-cache' \
  -H 'Postman-Token: ab6f2fde-9798-4215-939b-b3361fda9691' \
  -H 'X-Edx-Api-Key: PUT_YOUR_API_KEY_HERE'

DELETE (adjust user ids in last line):

curl -X DELETE \
  http://localhost:8000/api/server/organizations/1/users/ \
  -H 'Cache-Control: no-cache' \
  -H 'Content-Type: application/json' \
  -H 'Postman-Token: f60a5b25-d0d6-4f76-974f-3f649f184f62' \
  -H 'X-Edx-Api-Key: PUT_YOUR_API_KEY_HERE' \
  -H 'content-type: multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW' \
  -F 'users=9,5,1'

• checkout into branch of this PR and also make a couple of the GET and DELETE requests from above

Check the results:

• access localhost:8000/silk and see difference in number of queries and time on edx-solutions:master and open-craft:agrendalath/MCKIN-8417-optimize-OrganizationsViewSet.users-API; you can also analyze the SQL tab under each request

[UmanShahzad]

👍

• I tested this: works as expected from test instructions.
• I read through the code
• I checked for accessibility issues
• Includes documentation