Improve AdvertisementService

[Mat-san]

Overview

Right now, there's a few problems with it:

• Ad service methods are hard to understand.
• They tend to fail in unpredictible manners.
• They don't respect the model diagram.

Fix

To fix those problems, we plan to:

• Add the missing association between the advertisement and the pet.
• Change that association to 1-to-1.
• Redo the various methods using that new association.
• Change the AdvertisementDTO accordingly.
  • Change the petID attribute to a petDTO attribute.
• Update the unit tests accordingly.