Increase font size of nav elements for collapsed nav

[timhoffm]

We could afford slightly larger font and spacing here:

Rationale:

• The collapsed nav bar is used on small screens (mobile), where hitting the elements is a bit more difficult. Larger nav elements would make that easier.
• The increased size would have no disadvantage: The nav menu is only shown temporarily. When shown, there’s plenty of horizontal width, because each element is on its own row. There are not that many elements that there’s a danger they don’t fit on the screen vertically.

[timhoffm]

Just saw this got better though #834