Add OutputPage::wrapWikiTextAsInterface() to safely wrap wikitext

Add OutputPage::wrapWikiTextAsInterface() to safely wrap wikitext

This patch introduces a new method,
OutputPage::wrapWikiTextAsInterface(), which wraps the result of
OutputPage::addWikiTextAsInterface() in a tidy and robust way that
won't break if the wrapped message contains double newlines, extra
</div>, or other nasties.

This replaces a common unsafe pattern:
  $output->addWikiText( '<div class="..."' .... '</div>' );

Bug: T205624
Change-Id: I1040c7cf0ec1f5c4bef7c06d4486f50d85f2dc0f