Fix authentication helper so it works with newer versions of Django (Django 2+)

[Kami]

I installed recommended version of Django (aka one which is specified in requirements.txt), yet the /result/add/json/ API endpoint returns an error when enabling authentication for that API endpoint (setting ALLOW_ANONYMOUS_POST config value to False).

Here is the error / exception:

Traceback (most recent call last):
  File "/home/kami/.virtualenvs/codespeed-py3/lib/python3.7/site-packages/django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "/home/kami/.virtualenvs/codespeed-py3/lib/python3.7/site-packages/django/core/handlers/base.py", line 126, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/home/kami/.virtualenvs/codespeed-py3/lib/python3.7/site-packages/django/core/handlers/base.py", line 124, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/kami/.virtualenvs/codespeed-py3/lib/python3.7/site-packages/django/views/decorators/csrf.py", line 54, in wrapped_view
    return view_func(*args, **kwargs)
  File "/home/kami/.virtualenvs/codespeed-py3/lib/python3.7/site-packages/django/views/decorators/http.py", line 40, in inner
    return func(request, *args, **kwargs)
  File "/home/kami/.virtualenvs/codespeed-py3/lib/python3.7/site-packages/codespeed/auth.py", line 36, in _decorator
    elif hasattr(request, 'user') and request.user.is_authenticated():
TypeError: 'bool' object is not callable
"POST /result/add/json/ HTTP/1.1" 500 86983

Environment:

$ python --version
Python 3.7.6
$ pip show django
Name: Django
Version: 2.1.15
Summary: A high-level Python Web framework that encourages rapid development and clean, pragmatic design.
Home-page: https://www.djangoproject.com/
Author: Django Software Foundation
Author-email: foundation@djangoproject.com
License: BSD
Location: /home/kami/.virtualenvs/codespeed-py3/lib/python3.7/site-packages
Requires: pytz
Required-by: codespeed

Looking at the Django changelog, this behavior (method to attribute / property) changed in Django 2 which is installed by CodeSpeed by default (as per requirements.txt).

I assume this issue was never caught because no one who had authentication enabled ran CodeSpeed under Django 2+(?).

This pull request updates the authentication helper so it works with both versions of Django - one where that variable is a method (old versions) and one where it's a property (newer versions).

TODO

• Tests

[Kami]

I've added some mock based test cases in bed3f63.

[Kami]

@tobami When you get a chance, please let me know if there is anything else I need to do.

Thanks!