This invokes Parsoid to convert likely wikitext pastes.
This addresses about 20% of the <nowiki> insertion quantified at
https://fr.wikipedia.org/wiki/Wikip%C3%A9dia:%C3%89diteurVisuel/Avis/Nowiki
for 2015-04-08 which were due to inadvertent pastes of wikitext.
For a better user experience, suppress Parsoid's P-wrapping if the
output consists of a single top-level P element. This ensures that
pasting '[[Foo]]' won't add an unexpected newline before the paste.
Bug: T54204
Bug: T109449
Change-Id: I26a4cd8dc5b7e7caf16ca081dbe7baf6a7db8e5c
