Subnav UI improvements

[samuelsidler]

There are a few things in the sub-navigation that needs to be cleaned up:

• Desktop: The search button is no longer aligned with search input field.
• Responsive: On the homepage, when subnav is open, the star count appears on a higher z-index level than the subnav.
• Responsive: The current <a> area only includes the text itself. On mobile, they can make for a very small tap-target. Ideally, it's expanded to include the entire <li>.
• Responsive: Items in the subnav should align on the left with the title of the site ("Plugins"), including the search box. Should be a padding-left fix.

[obenland]

In 6425:

Plugins: Improve subnav UI on small sceens

Props samuelsidler.
Fixes #1823.