use created_at column for submission sorting

[sylvesterdamgaard]

Submissions are not sortable due to this query which expects the date to be in the data.
This issue is a regression from #306 where the date field is grabbed from the created_at column instead.

Result is that old submissions are sortable due to having a date property in data, but new entries do not.

Current query.
select * from form_submissions where form = 'tip_os' order by json_unquote(json_extract(data, '$."date"')) desc limit 100 offset 0

This PR changes the query builder to use the created_at column for sorting

The new query will look like this:
select * from form_submissions where form = 'tip_os' order by created_at desc limit 100 offset 0