Html::srcSet: allow density to be specified either with or without trailing 'x'

$wgLogoHD is meant to contain high-density alternatives for $wgLogo, but its
keys include the trailing 'x' (e.g., '1.5x'), making it unusable with
Html::srcSet(). Fix that by normalizing all density values to have a
single trailing 'x'.

Change-Id: I62cc3a9e4aeff3a7cb102de2965b8b40fd106c37