Replace get_user_model calls with Member

[0xAda]

Resolves an item of #229

[codecov]

Codecov Report

Merging #281 (5446c75) into master (45ee449) will decrease coverage by 0.00%.
The diff coverage is 99.32%.

@@            Coverage Diff             @@
##           master     #281      +/-   ##
==========================================
- Coverage   96.88%   96.87%   -0.01%     
==========================================
  Files         111      111              
  Lines        5131     5125       -6     
  Branches      281      281              
==========================================
- Hits         4971     4965       -6     
  Misses        128      128              
  Partials       32       32              

Impacted Files	Coverage Δ
src/authentication/providers.py	100.00% <ø> (ø)
src/challenge/views.py	84.41% <75.00%> (ø)
src/authentication/basic_auth.py	100.00% <100.00%> (ø)
src/authentication/serializers.py	100.00% <100.00%> (ø)
src/authentication/tests.py	100.00% <100.00%> (ø)
src/authentication/views.py	100.00% <100.00%> (ø)
src/backend/backends.py	100.00% <100.00%> (ø)
src/challenge/models.py	100.00% <100.00%> (ø)
src/challenge/tests/test_utils.py	100.00% <100.00%> (ø)
src/challenge/tests/test_views.py	100.00% <100.00%> (ø)
... and 17 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 45ee449...5446c75. Read the comment docs.