This changes TemplatesOnThisPageFormatter to use LinkBatch
and LinkBatch/LinkCache to manage the field.
Change-Id: I523158cdffc599d4d29bab91c98e55085130cee2
Linker::formatTemplates() was a static function that depended upon
global state like $wgLang (explicitly), $wgUser & $wgTitle (implicitly).
Moving it to a separate class allows us to clean it up a little bit and
use modern things like RequestContext and LinkRenderer.
Bug: T145177
Change-Id: Icdea8a2b299b4876feb3df3d66df3e4c104dd928