More padding around nav bar links

The nav bar links are spaced like they're all part of the same paragraph. Given some of their long and title-cased names, I honestly though some of them were wrapping around to two lines, but actually it's two separate links.

Adding a little more vertical whitespace would distinguish each link, and give them larger click/touch targets that I think would be more convenient. Here's how it looks with `.vp-navbar-dropdown-item a { line-height: 2em !important; }`:

Find the stylesheet or theme entry that controls `.vp-navbar-dropdown-item a`, then inspect the existing navbar link spacing. Apply the requested additional vertical whitespace and verify the dropdown links are visually distinct with larger click/touch targets at both normal and wrapped text widths.

索引モデルが issue の本文から書いたものです。