Fix: Combat issues

[PedroChaparro]

Fix new issues from #108.

• New fields on UPDATE_USER_LOOMIE_HP and UPDATE_GYM_LOOMIE_HP to send the attack value and notify if it was critical.
• Check the user and the gym both has Loomies before generating the combat token.
• Send the amount of alive player and gym Loomies in some messages (Combat start, use item, Loomies weakened)
• Custom message to confirm the usage of an item sending it's id and serial.
• Custom messages when an item cannot be used in a Loomie (Healing items).
• Send if user owns the gym on endpoint GET gyms/{id}.