Show validation results in the model editor

[robertbrignull]

Implements displaying validation in the model editor. Each validation result is added as a span 5 grid cell, so it sits underneath the modelings.

I wasn't sure whether the validation errors should span the full width or sit only underneath the modelings. I went with just under the modelings, but changing this would be very easy to do now or in the future.

For now I've disabled the "click on error to highlight model" functionality. I think it's less useful here than it is for the method modeling panel because all the modelings are visible at once and it's not that the invalid bit is on a different page. If we want to we could add highlighting of individual modelings, but it would add a little complexity.

Screenshot:

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[robertbrignull]

I can add some tests, but this is the general idea of how the UI will look and behave.

[koesie10]

LGTM, but adding a few tests and a story showing the validation errors would definitely be useful

[koesie10]

It would be nice if we were able to highlight the model on which the error is located, but this can still be added in a follow-up PR if this is something that comes out of a design review.

[robertbrignull]

I've opened a separate issue for this so we don't forget it.