Finalise helm chart, productionise frontend config, optional backend serving

[spwoodcock]

• Better align the helm chart with drone-tm frontend deployment pattern: (1) bundled frontend, served via backend (2) cloudfront + s3 for best prod setup.
• Slim the frontend img a little.
• Add config for serving the frontend via Django + Whitenoise (same as hotosm/website approach).
• Also replaced the custom image build pipelines with the reusable one - I disabled multi-arch for now as we bundle tippecanoe and need to handle that in a multi-arch compatible way first.

Documented process here: https://docs.hotosm.org/devops/deployment-process/

And instructions for manual AWS cloudfront invalidation role here: https://docs.hotosm.org/devops/cloudfront-s3-frontends/

With drone-tm and hotosm/website following these patterns (staging + prod deploy), I'm hoping we can roll this out to all apps (feedback / critique welcomed!)

[dakotabenjamin]

LGTM