(CE-565) Remove anon SiteWideMessages from page

Load Anon SiteWideMessages via AJAX instead, simplify things and add
some light caching. This should have the added benefit of slightly
better performance for anon page loads until we get to the new version
of SWM.

This is a quick temp solution for https://wikia-inc.atlassian.net/browse/CE-565.

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