Revert merge of DismissableSiteNotice into the core (r41679 and subsequent edits...

Revert merge of DismissableSiteNotice into the core (r41679 and subsequent edits). We should have higher standards than this for core code. This is largely my own crap code and there's a reason I didn't commit it to the core in the first place.
* The sitenotice_id hack is virtually unusable and needs to be replaced with a dedicated message update interface.
* There's a need for automatic message expiry.
* The Wikimedia-specific "spite the anons" feature, preventing anonymous users from dismissing site notices because they allegedly don't contribute to the wiki enough to deserve it, needs to be made optional.