CI: add check to avoid accidentally commiting images or large files
We plan to move all images to an external store (see #877 (closed)), so they should no longer be commited to the integration branches (main/release). Propose to add a check in the .pre
stage to confirm that no files with common image extensions, nor any particularly large file, is being added. It's important to automate this check as there's no way to recover the space from the error without rewriting the history of integration branches, which we very much don't want to do.