Change default styling for horizontal rule (hr) margin from 24px 0 to 1rem 0 🤔

Proposal

@peterhegman commented the following here:

I wonder if we want to look into changing the default hr margin from 24px 0 to 1rem 0 🤔

Would make the spacing of the headers a little more consistent (could mess up other areas though):

Before

After

And also this comment by @dmishunov :

That's a good point, @peterhegman! However, I would assume this should be tackled from the UX side of the things to match the Pajamas' scale and involving thorough visual testing of all hr instances. We might end up removing the hrs all together and rely on borders instead at some point. But that's the whole other story :)

This is related to the current need to remove the top margin from any HR which is directly below a page title.

Related Topic: Color

@dmishunov also made this comment regarding different coloring (as well as the spacing) for HR tags in issues vs. wiki.

We should consider standardizing that as part of this issue, or else opening a follow-on issue to address it.