refactor views to provide serializer_valid and serializer_invalid as …

[ralokt]

…hooks for subclasses.

I'm using this app for a project of my own, and wanted to set user.last_login.

I felt like what I had to do to accomplish this wasn't very DRY, as I had to re-serialize the request data in order to get to the user.

So I decided to factor that out in a manner reminicient of the form_valid and form_invalid methods of Django's generic views.

[codecov]

Codecov Report

Merging #413 into master will decrease coverage by 0.28%.
The diff coverage is 76.92%.

@@            Coverage Diff             @@
##           master     #413      +/-   ##
==========================================
- Coverage   90.67%   90.38%   -0.29%     
==========================================
  Files          14       12       -2     
  Lines         847      822      -25     
  Branches       29       29              
==========================================
- Hits          768      743      -25     
  Misses         66       66              
  Partials       13       13

Flag	Coverage Δ
#codecov	90.38% <76.92%> (-0.29%)	⬇️
#dj110	87.1% <76.92%> (-0.39%)	⬇️
#dj111	87.1% <76.92%> (-0.39%)	⬇️
#dj18	89.53% <76.92%> (-0.31%)	⬇️
#dj19	89.53% <76.92%> (-0.31%)	⬇️
#drf31	89.53% <76.92%> (-0.31%)	⬇️
#drf32	89.53% <76.92%> (-0.31%)	⬇️
#drf33	89.53% <76.92%> (-0.31%)	⬇️
#drf34	90.38% <76.92%> (-0.29%)	⬇️
#drf35	90.02% <76.92%> (-0.3%)	⬇️
#drf36	90.02% <76.92%> (-0.3%)	⬇️
#py27	90.38% <76.92%> (-0.29%)	⬇️
#py33	89.17% <76.92%> (-0.32%)	⬇️
#py34	90.02% <76.92%> (+0.53%)	⬆️
#py35	87.1% <76.92%> (?)
#py36	87.1% <76.92%> (?)

Impacted Files	Coverage Δ
rest_framework_jwt/views.py	93.18% <76.92%> (+0.68%)	⬆️
rest_framework_jwt/utils.py
rest_framework_jwt/models.py

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[ralokt]

I'm aware that this pull request decreases code coverage as currently measured.

However, this is the case because of refactoring for flexibility, not because I introduce any new behavior; I looked at the existing tests, they all seem to test views as a whole, and I don't see how I could contribute tests that cover my changes (although I'm open to suggestions!).

Also, #410 is evidence that I'm not alone in wanting something like this.