Latex improvements

[t-makaro]

The follow up PR to #992.

• Slightly better title spacing. (The gap between title/author/date is smaller. Added usepackage{titling}))
• Better spacing between cells. (Removed \tcbset{nobeforeafter})
• Paragraphs are no longer auto-indented like in markdown. (Added \usepackage{parskip})
• Better prompt font, and spacing (using tcolorbox's lengths)
• Fixes Hidden \includegraphics[]{} latex call #340. I was unable to use the \setkeys option to fix this, since it had a side effect of shrinking adjustimage boxes. However, by adding the Export flag to the adjustbox package it exports a new version of includegraphics that still allows use of all the keys whilst still setting a maximum size for images.
• Stop alt-text from being used as an image caption. Apparently, before, it was just removing the caption label but not the caption as the comments described. This PR now supersedes [WIP] Strict placement of images #990, and images in the pdf are now placed like in the notebook.

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[t-makaro]

Also: Spacing between cells:
Before:

After:

This applies to text before/after cells too.

CC @mgeier

[MSeal]

It looks good to me, but I have a hard to evaluating for potential latex cornercases. What's the risk factor in your opinion? Should we get another more latex knowledgeable review or do you feel pretty confident this is the right approach? If it's the later I can merge this now.

[t-makaro]

I believe it is the right approach.

The adjustbox package has an ability to add features to the original includegraphics (with a lower case export instead of Export), but the documentation explicitly warns that not all features may work with that method. This should actually be more feature filled and customizable than the original includegraphics.

All of the options in the original includegraphics should still work fine. I tested setting the width key to larger than the 90% maximum that I've set, and it will ignore the maximum if asked too which is good for people using custom templates.

My bigger worry is if someone made a template that used \Oldincludegraphics. This would break their template. I could re-add the \Oldincludegraphics definition for compatibility.

[MSeal]

Ahh the \Oldincludegraphics might be a valid concern. I've seen a lot of threads over the past two releases where people reference that in relation to other issue.

We could keep the backwards compatibility for the next patch release but note in the release notes that we'll remove that compatibility in 6.x?

[t-makaro]

@MSeal I managed to squeeze in one more fix. It's good to merge once tests pass.

[meeseeksmachine]

This pull request has been mentioned on Jupyter Community Forum. There might be relevant details there:

https://discourse.jupyter.org/t/nbconvert-5-6-0-release/1867/1