The places we were using `hstore_accessor` have now been converted to
Postgres JSON fields. Thus we no longer need `hstore_accessor` in order
to add accessors for the fields.
Instead we can use Rails' built-in support for Postgres JSON fields and
create accessors for our JSON hash keys with `store_accessor`. We get
rid of the data types because `store_accessor` doesn't work like that
and JSON already types our values with primitive data types.
Finally, add some extra validation tests now that the vaildation for our
JSON fields should work.
Refs #5316