Support longer user names in nav

Candy currently breaks if a user name is too long as it tries to fit in on a single line and it covers the content area.

What I have done is set a fixed width of 250px for the desktop nav, reduced the font-size on mobile, and properly aligned the user name with the other nav links.

Also, the name now has the same font-size and line-height of the nav button on mobile, so it looks really nice.