Extension:MassMessage/Technical debt cleanup

MassMessage is 4+ years old now, and technical debt has accumulated that needs to be cleaned up.

• Put the extension into a PHP namespace (MediaWiki\MassMessage)
• Have "targets" use a value object instead of passing arrays around
• Turn MassMessageTargets::getTargets() into an abstract MassMessage\SpamlistLookup or something, and have individual classes extend it
• Break down the giant MassMessage class into smaller ones that have less responsibility
• Allow using Wikidata items as spam lists phab:T171617
• Add missing filter name messages phab:T169824 (microtask)
• Fix invalid argument supplied for foreach phab:T162734 (microtask)
• Warn when <ref> tag is present but no <references> tag is present phab:T156167
• (there's probably more bugs that could be addressed, but I think this is a good scope for now)