Improve readability of sticky header in appendix

[matthijsgroen]

The header of the appendix table was transparent, making the text in the header hard to read when scrolling

Fixes #1110

[matthijsgroen]

@mlinksva is this something you could verify?

[mlinksva]

This does look nicer but it appears to me only the first column is affected:

It isn't immediately clear to me why, any idea?

[matthijsgroen]

It seems to be caused by:

https://github.com/github/choosealicense.com/blob/gh-pages/assets/vendor/hint.css/hint.css#L27

The position relative moves the dots out of the page flow and cause them to render on top of the sticky header.

Since this is a global stylesheet, I cannot forsee the impact of removing this position: relative for the rest of the site. It looks good without the position relative for the appendix page though.

Update: I just checked on other pages, but there the positioning of a hint following the dot will break.
They also work with really high z-index values. So I added a commit to set a z-index for this sticky header as well

[matthijsgroen]

The z-index value is now based on

choosealicense.com/assets/vendor/hint.css/hint.css

Line 62 in 3f5208a

z-index: 1000001; }

[mlinksva]

Tests are failing for unrelated reasons, will fix in a separate PR.