Update get_user_id to get_user since it is removed after django 2.1

[jxltom]

get_user_id is removed after django 2.1. Refer to django/django@fe99fb8

So update get_user_id to get_user in tests to make saleor compatible with django which version are newer than 2.1

Pull Request Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. The changes are tested.
6. The code is documented (docstrings, project documentation).

[codecov]

Codecov Report

Merging #2820 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2820   +/-   ##
=======================================
  Coverage   89.32%   89.32%           
=======================================
  Files         207      207           
  Lines       10489    10489           
  Branches     1011     1011           
=======================================
  Hits         9369     9369           
  Misses        803      803           
  Partials      317      317

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