Improve template whitespacing

[t-makaro]

Fixes #1059, and fixes #376. I chose this method because I noticed that the markdown exporter has the same issue which is problematic for people that need a title as the first thing in a markdown export.

I also made some improves to enhance readability of the latex exporter, but nothing will render differently due to this.

I also added a default documentclass to the base.tplx so that all of the style_*.tplx files can now be used on there own. This technically makes article.tplx redundant.

[MSeal]

LGTM, should we add a test for setting strip_preceding_newlines? Do you envision that field needing to ever be False?

[t-makaro]

I honestly can't envision strip_preceding_newlines ever needing to be false, but I thought that I'd add some way to disable it in case someone needs the old behaviour. In the interest of not adding excess config, it would probably be better to remove that check. If anyone ever needs it, we can add it later (it would also be possible to re-add the space in an inherited exporter with some work).

[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