Enlarge and move Jenkins head icon

Enlarge and move Jenkins head image a bit to be flush with the bottom of
the header bar.
Refactor CSS (top > bottom align)

Refactor CSS to make header images relative to the bottom of the header
bar rather than relative to the top.
Merge remote-tracking branch 'upstream/master'
Merge pull request #1360 from sfarbota/master

Enlarge and move Jenkins head icon