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.