Delete entities / Manage entity status #59

Create endpoints for all entities to be able to delete/disable them.

All delete methods should validate that the entity isn't used anywhere.

Whether we really want to delete from the database or not might be a different topic.
I suggest that we only allow "trashed" entities for now.

Might be nice to create a status field for each entity in which we store the current status.
Example:

0: disabled
1: enabled (default)
2: trashed?

See also visibility: #79