Add API endpoint for pattern rendering via POST

[thibaudcolas]

Description

Addresses #104. This adds an API endpoint built into the pattern library, and changes how tag overrides are loaded. This is so they can be loaded from the API data, rather than always loading the YAML file.

I could use additional feedback on this before adding the tests.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have added an appropriate CHANGELOG entry

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To test this:

 echo '{"template_name": "patterns/molecules/button/button.html", "config": {"context": {"target_page": {"title": "API"}}, "tags": {"pageurl":{"target_page":{"raw": "/hello-api"}}}}}' | curl -d @- http://localhost:8000/pattern-library/api/v1/render-pattern

Suggested CHANGELOG entry:

### Added

- New `/api/v1/render-pattern` [API endpoint to render patterns](https://torchbox.github.io/django-pattern-library/recipes/api-rendering/) via POST requests, with the pattern’s context and tag overrides as a JSON body ([#168](https://github.com/torchbox/django-pattern-library/pull/168)).

[bcdickinson]

I think there's some subtle things that need changing in render_pattern(), and it would also be good to see some tests that call the API and check this is functioning as intended, but this look 95% good to me.

[thibaudcolas]

Code changes from feedback: check. Tests: check. Docs: check! Re-running black and isort three times: check.