Improve handling of sorting in MongoDB backend

[ml-evs]

Currently all fields are sort-enabled by default, and the entire ?sort= parameter is passed through to the backend. We need to wrap this in some code to handle the following cases:

• Explicitly disallowed sorting, e.g. the model/schema has been changed to disable sorting on a field.
• If sorting a large number of results on non-indexed fields, catch any pymongo errors and return a useful error message.

It could be generally useful to allow sorting to be configurable (by list of allowed field names, and whether to allow multiple sort keys), to better support other backends too.