:bug: Caching fixes

[stevenbal]

Changes:

• Remove duplicate name SessionRefresh in middleware.py
• Fix caching issues caused by calling OpenIDConnectConfig.get_solo() in OIDCAuthenticationBackend.__init__ and cache the config on instances of the view, backend and middleware

[codecov-commenter]

Codecov Report

Merging #29 (3934c43) into master (ab5c9ec) will decrease coverage by 2.28%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
- Coverage   94.32%   92.04%   -2.29%     
==========================================
  Files          22       22              
  Lines         582      503      -79     
==========================================
- Hits          549      463      -86     
- Misses         33       40       +7     

Impacted Files	Coverage Δ
mozilla_django_oidc_db/backends.py	78.75% <100.00%> (+1.11%)	:arrow_up:
mozilla_django_oidc_db/middleware.py	88.88% <100.00%> (-1.12%)	:arrow_down:
mozilla_django_oidc_db/mixins.py	100.00% <100.00%> (ø)
tests/test_backend.py	100.00% <100.00%> (ø)
tests/test_middleware.py	100.00% <100.00%> (ø)
tests/test_settings.py	100.00% <100.00%> (ø)
mozilla_django_oidc_db/models.py	73.17% <0.00%> (-8.54%)	:arrow_down:

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[stevenbal]

Added the regression test for the caching issue here: https://github.com/maykinmedia/mozilla-django-oidc-db/pull/29/files#diff-31e088e0e9d455d2164fd6251e7c23a77afc5561b3746cdf83e08d6a05142154R391

As well as a test for the SessionRefresh caching: https://github.com/maykinmedia/mozilla-django-oidc-db/pull/29/files#diff-d319080c452f8bf63a43eda499db0c36b519a06613cfab6d97dacaab811fd257R27