Enable support for window functions

[timnyborg]

It seems that while this backend can support window functions out of the box, the flag hasn't been set, so a NotSupportError is thrown (see ESSolutions/django-mssql-backend#67)

I've just tried some common uses (rank over a partition, average over a row range frame), and they worked perfectly.

[absci]

There're few unit tests in expressions_window.tests.WindowFunctionTests failed. So I think it's better to fix those before enabling the feature.
For example:

ERROR: test_row_number_no_ordering (expressions_window.tests.WindowFunctionTests)
The row number window function computes the number based on the order
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vsts/work/1/s/.tox/py37-django22/lib/python3.7/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "/home/vsts/work/1/s/.tox/py37-django22/lib/python3.7/site-packages/mssql/base.py", line 565, in execute
    return self.cursor.execute(sql, params)
pyodbc.ProgrammingError: ('42000', "[42000] [Microsoft][ODBC Driver 17 for SQL Server][SQL Server]The function 'ROW_NUMBER' must have an OVER clause with ORDER BY. (4112) (SQLExecDirectW)")

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/vsts/work/1/s/django/tests/expressions_window/tests.py", line 156, in test_row_number_no_ordering
    ], lambda entry: (entry.name, entry.department, entry.row_number))
  File "/home/vsts/work/1/s/.tox/py37-django22/lib/python3.7/site-packages/django/test/testcases.py", line 1047, in assertQuerysetEqual
    items = map(transform, qs)
  File "/home/vsts/work/1/s/.tox/py37-django22/lib/python3.7/site-packages/django/db/models/query.py", line 274, in __iter__
    self._fetch_all()
  File "/home/vsts/work/1/s/.tox/py37-django22/lib/python3.7/site-packages/django/db/models/query.py", line 1242, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File "/home/vsts/work/1/s/.tox/py37-django22/lib/python3.7/site-packages/django/db/models/query.py", line 55, in __iter__
    results = compiler.execute_sql(chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size)
  File "/home/vsts/work/1/s/.tox/py37-django22/lib/python3.7/site-packages/django/db/models/sql/compiler.py", line 1142, in execute_sql
    cursor.execute(sql, params)
  File "/home/vsts/work/1/s/.tox/py37-django22/lib/python3.7/site-packages/django/db/backends/utils.py", line 67, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "/home/vsts/work/1/s/.tox/py37-django22/lib/python3.7/site-packages/django/db/backends/utils.py", line 76, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "/home/vsts/work/1/s/.tox/py37-django22/lib/python3.7/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "/home/vsts/work/1/s/.tox/py37-django22/lib/python3.7/site-packages/django/db/utils.py", line 89, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "/home/vsts/work/1/s/.tox/py37-django22/lib/python3.7/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "/home/vsts/work/1/s/.tox/py37-django22/lib/python3.7/site-packages/mssql/base.py", line 565, in execute
    return self.cursor.execute(sql, params)
django.db.utils.ProgrammingError: ('42000', "[42000] [Microsoft][ODBC Driver 17 for SQL Server][SQL Server]The function 'ROW_NUMBER' must have an OVER clause with ORDER BY. (4112) (SQLExecDirectW)")

[ghost]

All CLA requirements met.

[timnyborg]

Thanks. I see the contribution guide now has instructions for running the test suite, so I'll take a look.

[timnyborg]

Now passes the test suite. NthValue has been marked as unimplemented, and its 2 tests excluded. Window functions order by (select null) if no order_by is provided.