Lint and block too-large images

(or provide some complicated javascript tile based image process) because https://training.galaxyproject.org/training-material/topics/microbiome/tutorials/pathogen-detection-from-nanopore-foodborne-data/images/heatmap2.png is too large at 28000px square

- in chrome it didn't load for me, just showed 'broken'
- in firefox it loads after like a full minute, just to show this:


Start with the referenced heatmap2.png URL and inspect the repository for existing image or content-validation entry points; the issue names no files or tests. Done means the oversized image is detected and blocked, or an agreed tile-based alternative is provided, with the result checked in Chrome and Firefox.

Written by the indexing model from the issue text.