Soft deprecate QuickTemplate::msgWiki()

Soft deprecate QuickTemplate::msgWiki()

This method just should not exist. It does have a bad name that does not
say much about what it does and how it is supposed to be used. And it
does horrible things: it accesses the global $wgOut, which currently
results in test failures, see
https://integration.wikimedia.org/ci/job/quibble-vendor-mysql-php70-docker/15200/console

Note how it was marked as @private, which is honorable, but doesn't
work for multiple reasons. First, an overload exists in the BaseTemplate
subclass. Second, calls from external code exist. Third, if it would be
really private, it could as well be deleted, because there is no private
caller.

Luckily the number of callers is extremely close to zero, see
https://codesearch.wmflabs.org/search/?q=>msgWiki%5C(

A patch for the Collection extension already exists.

Change-Id: I8d2c14f10fbf314735d1aa82bdc8edfb4fa9a0dd