Fix cases where the datafile is huge

[ewjoachim]

When the json datafile is >1MB, the default mode of the Github content API is to skip the content and provide a link to it.

If we use header Accept: application/vnd.github.raw+json, the application will:

• Return the raw file instead of a payload (it's good, we only want the file) as long as it's <100MB
• Return Content-Type: application/vnd.github.raw+json

This PR:

• Adds code to the GitHub client to send custom headers (surprisingly, it's the first time we needed to do it)
• Add the said Accept header to the datafile function
• Setup the client so that the said Content-Type response header makes return text instead of bytes. (I'm not 100% sure it's what we should do but we can always revert this later).

[github-actions]

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[github-actions]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
coverage_comment
github_client.py
storage.py
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