Preserve sort order of list elements

[Darkin8]

Saving items with partially fetched fields from DB will lead to overwrite of 'sortOrder' field with max value for this field in the list.

Changed behavior:
check for 'sortOrder' field presence in schema.

[JedWatson]

Thanks for the fix @Darkin8!

Not sure why the build failed for your PR, but I added a sightly different implementation using keystone.mongoose instead of require('mongoose') to get Schema.Types.Number.

The reason for this is that you can provide a different instance of Mongoose to the one Keystone includes using the keystone.connect method, for edge-cases where the usual require logic fails to find a common package between Keystone and your project (e.g. running with npm link).