Check marking schemes for attachments to prevent leakage

[tuncbkose]

Fixes #1782. The problem of adding images to marking schemes remains unsolved.

[github-actions]

👈 Launch a Binder on branch AaltoSciComp/nbgrader/attachment_leakage

[brichet]

Thanks @tuncbkose

[tuncbkose]

Not sure why the docs have failed

[brichet]

Not sure why the docs have failed

Seems to be related to sphinx_rtd_theme not compatible with Sphinx>=7. I'm testing it locally and I'll fix it.
I don't know why this only happened yesterday.

EDIT: I opened #1791

[brichet]

Suggested change

	### BEGIN MARK SCHEME
	Text text text text text.
	
	Grade grade grade.
	
	Mark mark mark mark.
	### END MARK SCHEME
	### BEGIN MARK SCHEME
	Text text text text text.
	
	Grade grade grade.
	### END MARK SCHEME

Thanks @tuncbkose, I was more thinking about alternative text to the image, to test the regular expression.
But if you think it is relevant to keep also a test with 2 images inserted we can also keep it (maybe keeping text around is also a good thing)

[tuncbkose]

Thanks for the clarification. I'll add one with an alternative text. As for this example with two attachments and text around them, I don't think it matters too much but it doesn't hurt to keep it.

[brichet]

Failures are not related.

Thanks @tuncbkose