General cleanup of old issues

[sergei-maertens]

• Fixed #1 - schema generation code is now a mixin instead of a final class
• Fixed #6 - ensure the Content-Type header param is in the API schema. This removes confusion that you always need to send the Content-Type header on requests with a body
• Fixed #23 - use constants where possible
• Fixed #28 - allow permissions to be defined on HTTP methods if there is no action available (which is an extension done by viewsets)

[codecov-io]

Codecov Report

Merging #121 into master will increase coverage by 1.04%. The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
+ Coverage   46.57%   47.62%   +1.04%     
==========================================
  Files          63       63              
  Lines        2233     2249      +16     
  Branches      334      339       +5     
==========================================
+ Hits         1040     1071      +31     
+ Misses       1136     1124      -12     
+ Partials       57       54       -3

Impacted Files	Coverage Δ
...tifications/management/commands/register_kanaal.py	0% <0%> (ø)	:arrow_up:
vng_api_common/notifications/handlers.py	0% <0%> (ø)	:arrow_up:
vng_api_common/api/permissions.py	66.66% <100%> (ø)	:arrow_up:
vng_api_common/notifications/constants.py	100% <100%> (ø)	:arrow_up:
vng_api_common/caching/introspection.py	83.33% <100%> (+3.33%)	:arrow_up:
vng_api_common/inspectors/view.py	76.02% <100%> (+3.64%)	:arrow_up:
vng_api_common/middleware.py	58.45% <80%> (+6.13%)	:arrow_up:

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