Ensure all file handlers use utf-8 encoding.

[rtibbles]

Summary

• Sets explicit utf-8 encoding on logging file handlers
• Encodes messages to utf-8 before writing them as bytes to the stream for logging to terminal

References

Attempts to fix #6298

Reviewer guidance

Try running kolibri manage listchannels on a Windows machine with a non-Latin alphabet entitled channel.
Check the logs from Kolibri running and see if it is full of lots of errors from logging.

---

Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• 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]

Looks good to me @radinamatic - no encoding errors observed in the console and the logs after running kolibri manage listchannels:

Logs: logs.zip

[radinamatic]

Manual QA by @pcenov passed! 💯