Fix weird avatar alignment in header bar.

I noticed that the avatar in the header bar was overflowing the bar by a few
pixels in Chromium. Strangely, it underflowed in Firefox.

The solution seemed to be to set an explicit font-size for the text input and
then align the avatar to top instead of middle.

All manual, on Firefox and Chromium in Ubuntu and OSX. I don't have a Windows box / VM here to test out.