Fix: Add drop to bucket

* When adding a drop to a bucket, copy all the existing drop-specific
metadata (links, tags, places) over to the new BucketDrop

* Deleted DropsControllerTest. DropsController does not have any
endpoints for testing. These were removed following the change in
the strategy for adding metadata to drops
Implement Get/add/delete river and bucket drop comments

* The drop comments are also part of the data that are copied
over whenever a drop is added to a bucket