Add company v3 endpoints

[reupen]

This adds the v3 company and Companies House endpoints.

[codecov-io]

Codecov Report

Merging #252 into develop will increase coverage by 0.19%.
The diff coverage is 98.66%.

@@             Coverage Diff             @@
##           develop     #252      +/-   ##
===========================================
+ Coverage    92.56%   92.76%   +0.19%     
===========================================
  Files           56       56              
  Lines         1924     1990      +66     
  Branches       179      179              
===========================================
+ Hits          1781     1846      +65     
- Misses         121      122       +1     
  Partials        22       22

Impacted Files	Coverage Δ
datahub/core/mixins.py	100% <ø> (ø)	⬆️
datahub/company/views.py	100% <100%> (ø)	⬆️
datahub/core/serializers.py	91.83% <100%> (+0.34%)	⬆️
datahub/company/urls.py	100% <100%> (ø)	⬆️
datahub/company/serializers.py	97.81% <98.27%> (+0.19%)	⬆️

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[canni]

Hmm why change of related name?

[reupen]

@canni
@jim68000 asked for the endpoint to use children instead of subsidiaries. So I changed the related name to keep it consistent.