Generate ids for entities without auto-genarated @Id

* Added AbstractJpaDao#getLogSequeceNumber to generate
ids for entities that don't have db-generated ids.

* Changed the save method for Tag, Link and Place entities
to use EntityManager#merge. This is because the id is manually
set
Add @ResponseBody to delete methods

* Without the @ResponseBody annotation on the delete
methods, a 404 is returned to the client
Fix: Checking existence of metadata item before delete

* First check if the entity is specific to the user before proceeding
to create a new entry in the account_droplet_* tables with the
deleted flag switched on
Set email and name in GetCommentDTO account property

* The name and email are used by the client to display the
comment's author and avatar respectively
Fix logic bug

* If the metadata item being checked for exists and has not
been marked as deleted, mark as deleted and save. Otherwise,
if the item doesn't exist, create it as a "deleted" item.