Add, modify and delete river drop form field values implemented
Add, modify and delete river drop form field values implemented
addDropForm: Raise duplicate error when fom readded to drop.
Added an abstract DropForm

* RiverDropForm + BucketDropForm now inherit from this super class
* Drops will now have a list of DropForm populated accordingly
depending on whether it is a RiverDrop or BucketDrop