Implement add and remove drops from Bucket

* Added a new model, BucketDrop to break the many-to-many relationship
between drops and buckets. This entity is only used for adding a drop to a
bucket.

* Added a @Transient buckets property to the Drop model. This is a non-persistent
property for storing the buckets that a drop has been added to

* Added JpaDropDao#populateBuckets for fetching the buckets that a drop has been
added to