Index datasets table by user and created date

Usually when showing datasets we want to order by the created_at. With both values in the index this query can be done quicker.
Merge pull request #968 from theodi/feature-index-datasets-created-at

Speed up dashboard and owned dataset queries
Use precomputed value of certificate level

This call was loading in the entire response_set/question/answer tree just to render a single value that was already computed.
Use unambiguous column name
Simplify collection of available surveys to complete
Don’t need user check

Dashboard is only available to logged in users.
Use counter instead of passing object
Calculate expiring certificates in database
Load info for each response in one query instead of two.
Use helper method for this
Simpler grouping loop for questions
Only call all_autocompleted once
Remove unused variable
Move autocomplete_override_message to field on `Response`

An enormous amount of time was spent joining the `autocomplete_override_messages` table which was unnecessary as there should only be one message per field so it can be stored on the `Response`.

I’ve also renamed the field `explanation` as that was what it was being used for.
this is already a boolean