Switch to Django checks for config checking.

[alexmv]

Fixes:

How changes were tested:

Screenshots and screen captures:

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[timabbott]

What's the benefit of moving to the checks system?

[alexmv]

Better error messages, rather than an assert stack trace. They also run implicitly, in more places.

[timabbott]

Merged, thanks @alexmv!

[timabbott]

Backported for 11.5 in #36712.