Remove @todo in SpecialPage for making it into ContextSource

It is better to have $context as a member variable,
which can be accessed, $special->getContext(), as needed
with a composition approach instead of inheritance.

Having SpecialPage directly inherit makes testing code
a bit more challenging and messy.

Change-Id: I5e5587cbbc9abfb2fa4ab1e7603cd2ddcad1b96d