Metadata migration script

* Script for migrating data from the account_droplet_* tables
to the bucket_droplet_* and river_droplet_* tables. Since the account_droplet_*
tables have no information on the origin of the drops, i.e. river or bucket, the
data will be copied to all the new metadata tables
Refactor tests and DAO implementation classes

* Refactored the unit tests for links, places and tags.

* The hash and canonical name values are now set in
the DAO implementation classes. In this way, the tests
verify the generated hash against the expected value
Fix: Incorrect column mapping
Add missing unit and integration tests
Authentication when adding drop metadata

* Only collaborators and owners of a bucket can add metadata
to the drops within the bucket