Fix UnderlineNav overflow

[shawnbot]

This makes some tweaks to the UnderlineNav component:

1. Adds overflow-x: auto for horizontal scrolling, and overflow-y: hidden to prevent vertical scrolling.
2. Removes 1px clipping via margin-bottom: -1px on .UnderlineNav-body (because: why?)

I'm requesting reviews from folks who have lots of experience with this component. Thanks!

[shawnbot]

This appears to be working:

desktop	mobile

[ampinsk]

Just for context, the margin-bottom: -1px on .UnderlineNav-body was so that the orange line sat on top of the gray line, instead of right next to each other:

If it's causing problems then I don't think it's too much trouble to get rid of it! But is nice in theory 😄

[shawnbot]

@ampinsk thanks, that's helpful! I'm going to revert that commit.

[emilybrick]

This lgtm! Once this ships I will remove the custom css I shamefully added here 😬

[shawnbot]

Welp, this turns out not to have been a good change after all. overflow-y: hidden actually hides the lower half of the underline:

I tried a couple of things in the inspector and failed. I think getting the underline to overlap the border is going to require some more work, so in the meantime @ampinsk, are you okay with nixing the margin-bottom: -1px? 😬

[ampinsk]

Yeah that's fine! It's a small detail so if it's a big headache we should nix it 😄