[server] DELETE categories: handle entries of category to be deleted

[danielblagy]

• Add new parameter to DELETE v1/categories/:id called delete_entries (boolean).
  • Endpoint should look like this: v1/categories/:id?delete_entries=true.
• If delete_entries == true, delete entries with category_id=:id and user_id=username.
• If delete_entrues== false, set entries' category_id to NULL where category_id=:id and user_id=username.