Add header levels to CSS reset

[cchaos]

I thought we had this somewhere, and maybe it was only in Kibana at some point, but I think we should equal out all the h1 -> h6 tags in the CSS reset. Otherwise, if consumers use these tags outside of EuiTitle or EuiText they get the browser's default rendering.

Example usage

Someone placed an h2 tag inside a Vega tooltip so it's quite large:

[snide]

I dug into this a little more and there is an issue. I'll chat with @cchaos and we'll figure out the best way to handle it, but it will likely involve adjusting our reset file.