[JRO] Use polymorphic association w/topics in posts

Up until now we were using STI (single table inheritance) to reference topics and private topics. This commit moves us closer to a better designed API with topics and private_topics being their own, discrete models, and posts referencing either with a polymorphic reference called “postable”.

* Migrate data from one table, to two tables.
* Create a private_topic_mailer and views.
* Update rake and rspec-core
[JRO] Move posts#index to topics#show

TODO: Move similar to private_topics