* Add <ostatus:conversation /> tag to Atom input/output
Only uses ref attribute (not href) because href would be
the alternate link that's always included also.
Creates new conversation for every non-reply status. Carries
over conversation for every reply. Keeps remote URIs verbatim,
generates local URIs on the fly like the rest of them.
* Fix conversation migration
* More spec coverage for status before_create
* Prevent n+1 query when generating Atom with the new conversations
* Improve code style
* Remove redundant local variable
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