Cleanup popover arrows

Created by: MartijnCuppens

I moved and copied the .arrow's pseudos shared border-width: to the pseudo elements themselves. This will generate 4 more properties (oh no 😱), but will remove 16 selectors of which 8 attribute selectors (woohoo! 🎉). As you can see, this made my day.

I've also nested the pseudo's in the scss.