Allow interlanguage link prefixes that are not language codes

$wgExtraInterlanguageLinkPrefixes holds a list of interwiki prefixes to be
treated as language codes if $wgInterwikiMagic is true.

To set the display text for the interlanguage links generated by this
code, you need to create MediaWiki:Interlanguage-link-foo, where "foo" is
the interwiki prefix.  To provide a friendly site name for the link title
text, use MediaWiki:Interlanguage-link-sitename-foo.  On the WMF cluster,
these messages could be set using the WikimediaMessages extension.

Information about extra language links (in the site language only) is
provided via the API in meta=siteinfo&prop=interwikimap.

Bug: 32189
Change-Id: I3d04760e2d9fb3320bb71e3d5ad115eed54a899c
Merge "Allow interlanguage link prefixes that are not language codes"