Import constants from le-utils and add metadata translation mixin

[marcellamaki]

Summary

Description of the change(s) you made

Imports the metadata constants from le-utils and adds the strings from metadata that were used in kolibri.

Manual verification steps performed

Referenced both the constants and some strings to make sure they were available in vue files and was able to access both.

Does this introduce any tech-debt items?

Does adding this in the existing translation mixin file make sense, or would it be better to create a separate file for this? (Originally I added it separately, but then I thought that might be more confusing, if it was the only thing that was handled differently).

Contributor's Checklist

PR process:

• If this is an important user-facing change, PR or related issue the CHANGELOG label been added to this PR. Note: items with this label will be added to the CHANGELOG at a later time
• If this includes an internal dependency change, a link to the diff is provided
• The docs label has been added if this introduces a change that needs to be updated in the user docs?
• If any Python requirements have changed, the updated requirements.txt files also included in this PR
• Opportunities for using Google Analytics here are noted
• Migrations are safe for a large db

Studio-specifc:

• All user-facing strings are translated properly
• The notranslate class been added to elements that shouldn't be translated by Google Chrome's automatic translation feature (e.g. icons, user-generated text)
• All UI components are LTR and RTL compliant
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• Code is clean and well-commented
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Reviewer's Checklist

This section is for reviewers to fill out.

• 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

[sairina]

lgtm! If we need to add/update more in the future, we have this as the foundational work to use. Thank you so much!