Preserve sort order of list elements

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.
More appropriate usage of mongoose as keystone.mongoose.
Sync with original Keystone.
Types.LocalFiles fixed functionality on sort / upload / delete.
Merge pull request #649 from Darkin8/master

Types.LocalFiles fixed functionality.