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
Properly escape topic title in compare-post-revisions

Passing a topic title content directly into l10n results in the
topic title receiving magic word/template substitution.  Output
and use the l10n safe topic-of-post and post-of-summary properties
instead.

Change-Id: I12d07846d9f1c037b059c929e3a6a660dd5aacb2