add class for images with a drop shadow

[tobyhodges]

To address the issue described in #513, this introduces a img.image-with-shadow class with a drop shadow. If and when this is accepted, I'll document it in the lesson example.

[maxim-belkin]

@tobyhodges, would you mind updating the PR to have a single drop shadow only? Or is there a case where two similar shadows are useful? Also, what do you think about changing img.image-with-shadow to article img ?

[tobyhodges]

This fell off my radar, sorry. I have made a note to update the branch after the Core Team retreat is over (next week)

[tobyhodges]

@maxim-belkin updated to use a single, grey shadow as you proposed. I did not adjust the selector to apply this shadow to all images in the article because I suspect the shadow effect will be undesirable in many circumstances. IMO it is better as an "opt-in" feature, which the .image-with-shadow class will provide.

[maxim-belkin]

Thanks, Toby. Looks good. Current selector works when used like so:

![alt text](../fig/figure.png){: .image-with-shadow}
![alt text](../fig/figure2.png){: class="image-with-shadow"}

I have to admit that in my first attempt I did:

![alt text](../fig/figure.png)
{: .image-with-shadow}

which resulted in the following HTML structure: p[class="image-with-shadow"] > img. I know this is my mistake but I wonder if this is something we want to take care of. All we'd have to do is extend the selector to images within paragraphs with "image-with-shadow" class:

p.image-with-shadow img,
img.image-with-shadow { ...

[tobyhodges]

That is a great suggestion, @maxim-belkin, because I agree that many lesson authors are likely to try what you tried (it is also what my muscle memory tries to make me do every time!)

[tobyhodges]

Addressed in 4b20368

[maxim-belkin]

Looks great! 👍🏻

[maxim-belkin]

Thank you, Toby!