Add base_ordering to OrderingFilter

[kkirss]

base_ordering is an optional argument that allows you to specify a set of fields that will be given to order_by() after any provided ordering options. This is useful if you filter by a non-unique field but want an unique field to serve as a base. Django guarantees a consistent order only for querysets with at least one unique ordering field. Consistent ordering is important for things like pagination.

[codecov-io]

Codecov Report

Merging #1115 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1115      +/-   ##
==========================================
+ Coverage    98.1%   98.12%   +0.02%     
==========================================
  Files          15       15              
  Lines        1216     1229      +13     
==========================================
+ Hits         1193     1206      +13     
  Misses         23       23

Impacted Files	Coverage Δ
django_filters/filters.py	99.71% <100%> (+0.01%)	⬆️

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