Fix utf-8 encoding issues for all Python CSV exports

[rtibbles]

Summary

• Consolidate CSV file opening for writes.
• Uses utf-8-sig to mark BOM at beginning of file to ensure utf-8 decoding in Excel
  …

References

Fixes #8001

Reviewer guidance

• Export CSV files with unicode content
• Open in Excel on Windows
• Ensure that it properly opens with unicode encoding

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Testing checklist

• Contributor has fully tested the PR manually
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Reviewer checklist

• Automated test coverage is satisfactory
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• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[pcenov]

@rtibbles let me know when I can retest this after the additional code changes. As I mentioned earlier in Slack I didn't see any issues in Windows when opening .csv files with UTF content. On a side note there's one outstanding encoding related issue which I've reported here: #7978 - just want to bring it to your attention.

[rtibbles]

Thanks @pcenov - I'll take a quick look to see if I can resolve it.

[pcenov]

Retested today on both Win 10 and Ubuntu with the latest release candiate https://github.com/learningequality/kolibri/releases/tag/v0.14.7-rc4 - the .csv files with UTF content are exported correctly.