Code structure refactor

[matmair]

This PR implements #5578 and reorganises the code base more scalable. This is the first PR in a string of PRs needed, see #5578.

• move container things into contrib/docker
• move Django stuff into src/backend
• add docs about structure to CONTRIBUTING
• move misc things out
• test docker build
• test package build
• update docs for bare metal git install

closes #5578

Breaking Change

This is a significant breaking change as it rearranges a lot of source files. Any developers who are using the devcontainer setup will need to do a complete rebuild (without cache) of the devcontainer.

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[matmair]

@SchrodingersGat I can not find any usage of these CI checks:

• check_api_endpoint
• check_locale_files
  any idea why?

[SchrodingersGat]

@SchrodingersGat I can not find any usage of these CI checks:

• check_api_endpoint
• check_locale_files
  any idea why?

These are both outdated now:

• check_api_endpoint is now handled by invoke wait
• check_locale_files is now defunct since moving to crowdin

[SchrodingersGat]

@matmair in general this looks OK - what still to do? Any additional unit tests we would need to add?

[matmair]

@SchrodingersGat not really, I still need to add docs - should be good to go end of week.

[SchrodingersGat]

@matmair I have made some tweaks so that the devcontainer works again

[matmair]

Ok this seems to work with docker and devcontainer - the documentation issues are because of the already adapted github source links. Happy for another review; this has been very long running so not sure if I have missed something. Thank you for the help @SchrodingersGat

[SchrodingersGat]

Do we need to keep the empty requirements.txt in the top level directory?

[matmair]

In my tests the packaging mechanism stopped detecting that the project uses python if I removed it.

[SchrodingersGat]

@matmair I have reviewed locally again and it all seems to be working well. I will merge this in, however we should all be on the lookout for any issues stemming from this. This is a huge change to the codebase so there may well be problems.

The 0.15.x release target is for the end of the month, so let's take care until then.

[matmair]

@SchrodingersGat I expect some small surprises but most bases should be covered with the CI tests