Use get_by_natural_key in jwt_get_secret_key

[raverat]

As stated in commit a3b4d44d2fc34d7752793ccbade2684707bf41da, the field user_id in payload will be deprecated.

Instead of explicitly using user_id field to retrieve user in utils.jwt_get_secret_key function, we should use User.objects.get_by_natural_key with the value in the payload dictionary at username_field key.

[codecov]

Codecov Report

Merging #418 into master will decrease coverage by 0.33%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #418      +/-   ##
==========================================
- Coverage   90.67%   90.34%   -0.34%     
==========================================
  Files          14       12       -2     
  Lines         847      818      -29     
  Branches       29       29              
==========================================
- Hits          768      739      -29     
  Misses         66       66              
  Partials       13       13

Flag	Coverage Δ
#codecov	90.34% <ø> (-0.34%)	⬇️
#dj110	87.04% <ø> (-0.45%)	⬇️
#dj111	87.04% <ø> (-0.45%)	⬇️
#dj18	89.48% <ø> (-0.36%)	⬇️
#dj19	89.48% <ø> (-0.36%)	⬇️
#drf31	89.48% <ø> (-0.36%)	⬇️
#drf32	89.48% <ø> (-0.36%)	⬇️
#drf33	89.48% <ø> (-0.36%)	⬇️
#drf34	90.34% <ø> (-0.34%)	⬇️
#drf35	89.97% <ø> (-0.35%)	⬇️
#drf36	89.97% <ø> (-0.35%)	⬇️
#py27	90.34% <ø> (-0.34%)	⬇️
#py33	89.11% <ø> (-0.38%)	⬇️
#py34	89.97% <ø> (+0.48%)	⬆️
#py35	87.04% <ø> (?)
#py36	87.04% <ø> (?)

Impacted Files	Coverage Δ
rest_framework_jwt/models.py

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