frontpage: nicer layout for main navigation options (prefix and search)
frontpage: nicer layout for main navigation options (prefix and search)

- vertically align prefixes with the start of the search forms
- rename code search button to "Search code", for consistency with the
  analogous button in the header
- center horizontally the titles of prefixes and search forms

nonas/debian-qa,Debian/debsources,clemux/debsources,devoxel/debsources,zacchiro/debsources,oorestisime/debsources,Debian/debsources,matthieucan/debsources,sophiejjj/debsources,Debian/debsources,matthieucan/debsources,matthieucan/debsources,sophiejjj/debsources,nonas/debian-qa,zacchiro/debsources,nonas/debian-qa,vivekanand1101/debsources,zacchiro/debsources,devoxel/debsources,oorestisime/debsources,matthieucan/debsources,sophiejjj/debsources,zacchiro/debsources,sophiejjj/debsources,nonas/debian-qa,sophiejjj/debsources,clemux/debsources,oorestisime/debsources,oorestisime/debsources,oorestisime/debsources,clemux/debsources,devoxel/debsources,vivekanand1101/debsources,devoxel/debsources,vivekanand1101/debsources,Debian/debsources,clemux/debsources,clemux/debsources,devoxel/debsources,Debian/debsources,vivekanand1101/debsources,matthieucan/debsources,vivekanand1101/debsources,zacchiro/debsources