Add markdown cache columns to the database, but don't use them yet
This commit adds a number of _html columns and, with the exception of Note, starts updating them whenever the content of their partner fields changes. Note has a collision with the note_html attr_accessor; that will be fixed later A background worker for clearing these cache columns is also introduced - use `rake cache:clear` to set it off. You can clear the database or Redis caches separately by running `rake cache:clear:db` or `rake cache:clear:redis`, respectively.
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