/posts/:id/events incorrectly lists appeals as always being resolved.
This is because events UNION together appeals and flags, which doesn't
quite work because for appeals is_resolved is a method, not an
attribute. is_resolved was hardcoded to true so it'd work in the UNION.
This changes PostEvent to be a wrapper object around PostFlag /
PostAppeal, instead of a UNION. PostEvent delegates everything to the
inner flag/appeals object, so that is_resolved works correctly.
Also, this incidentally fixes a problem with /posts/:id/event.xml not
serializing correctly.
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