Add Translation.value and .properties, with data model contents

[eemeli]

Adds the JSON fields .value (required) and .properties (optional, default None) to Translation, and sets them by parsing the .string at all places where translation objects are created.

These fields are not (yet!) read anywhere, which also matches the corresponding Entity fields. The intent is to submit a follow-up PR that changes the REST API to pass these fields rather than .string for all entities and translations, in both requests and replies.

The data migration included here is... heavy. It needs to read, parse, and modify all Translation objects. That'll take a while even on a high-end machine.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 70.93023% with 50 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.93%. Comparing base (91be114) to head (a9d46cf).

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[mathjazz]

Thanks for the patch. The code looks good.

We need an explicit deployment plan, including:

• expected runtime on prod-ish data
• rollback/failure plan
• whether this should be split into schema migration + offline/backfill command instead of a blocking Django migration