Allow localisation of site link text

This uses the same system message infrastructure as is used for the
$wgExtraInterlanguageLinkPrefixes feature in core. It is fully backward
compatible.

Also adds a title attribute to each link, again using the same logic as
is used in core for interlanguage links.

Bug: 68359
Change-Id: Id6b1aac8e824859bfd6de3b4a8c7f020ce59579e