For each DetailedView subclass, we add a `warnings` array to:
1. The top-level response.
2. Each individual call under the `details` key.
We use the `.thresholds` hash on the DetailedView to determine what's a
warning. If that hash is empty (the default), then the warnings array
will always be empty.
This uses an ActiveRecord subscriber to get queries and calculate the
total query time from that. This means that the total will always be
consistent with the queries in the table. It does however mean that we
could potentially miss some queries that don't go through ActiveRecord.
Making this change also allows us to unify the response JSON a little
bit, making the frontend slightly simpler as a result.
