Deprecate SpecialPage::getTitle

Callers should use SpecialPage::getPageTitle, which is
exactly identical.

This is so that in the future we can turn SpecialPage
into a ContextSource, which requires getTitle to return
getContext()->getTitle.

Change-Id: Icdcf5d5295ef5e7f08b1d403e0c123f78738fd40
Merge "Deprecate SpecialPage::getTitle"