Don't treat topic title as wikitext

Topic titles inserted into l10n via plaintextSnippet were receiving
template substitution via Message::text().

This patch changes it arround to always output the l10n safe topic-of-post
property on topic titles so there is always a safe value to stick into
l10n when dealing with a topic title in a template.

Change-Id: I1ce7a49304abe87ec663c193461def4ee053273d
(cherry picked from commit 29d6daf34f2125ef48373621b25734b9bd87f076)
Merge "Don't treat topic title as wikitext" into wmf/1.24wmf22