Refactor backends to filters + bugfix

[n2ygk]

Fixes #459

Description of the Change

• renamed from backends to filters
• bugfix: camelcase, etc. sort parameters were being ignored.
• support for sorting via relationship paths.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• changelog entry added to CHANGELOG.md (in JSONAPIOrderingFilter #459)
• author name in AUTHORS

[codecov-io]

Codecov Report

Merging #464 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #464      +/-   ##
==========================================
+ Coverage   93.37%   93.44%   +0.07%     
==========================================
  Files          56       56              
  Lines        3183     3217      +34     
==========================================
+ Hits         2972     3006      +34     
  Misses        211      211

Impacted Files	Coverage Δ
example/settings/dev.py	100% <ø> (ø)	⬆️
rest_framework_json_api/filters.py	100% <100%> (ø)
example/views.py	92% <100%> (+0.1%)	⬆️
example/tests/test_filters.py	100% <100%> (ø)

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[sliverc]

Thanks. This looks good.