SpecialWhatlinkshere: Add autocompletion for target input

This adds the mw-searchInput class so suggestions are given by
mediawiki.searchSuggest (if loaded by the skin).

Bug: 65209
Change-Id: If4ae687ba125c002eedce114820a271cf615011b
Merge "SpecialWhatlinkshere: Add autocompletion for target input"