Figure caption width in tutorial

[jpeterson976]

Fixes #1762

I may have added a little too much to the new tutorial section, so let me know if I should cut it back at all.

I also changed max-width to text-width since that's a little more accurate, and made a minor CSS change for text width captions to better account for medium and large sizes.

[iturgeon]

The only argument I have against Text-width is when using captions on large images that extend past the text width. The image below shows the text width vs caption width under a large figure

[zachberry]

@jpeterson976 @iturgeon Both good points. max-width is used for Iframes for sizing which means beyond text width, so using max-width for figure width is probably a mistake. But there is the weird nuance that Ian brought up that text-width really means max-width when the figure size is large (In reality it seems there's no major difference for captionWidth when the image size is large).

Here's my thought - what if we stick with this change - text-width. But then disable the caption width select in the UI if the size is Large, and make a mention in the docs that this property has no effect if size is Large.

[zachberry]

Actually, it doesn't have any effect for size = medium either, since medium images already are at text width. So perhaps we disable the input for large and medium sizes.

[jpeterson976]

Works great in the editor with my testing. Code looks good too, found some stuff I missed in the original PR. Only change I can think of is the new tutorial section should probably be updated to mention the caption width option is only available for small and custom sizes.