Contracts endpoint

[smarbos]

Description:
Create a Contracts Linking an Animal and a Human.

Parameters:

• humanId (string)
• animalId (string)

Todo:

• I think we can use a BEFORE hook to check if both human and animal exists (and do any more check if we need) and then creating the contract. @anabellaspinelli what do you think?
• Define contract, maybe this should be a variable set by the shelter.

[anabellaspinelli]

Yes about checking existence before persisting. Hoes before broes.

About the "contract" variable:

• is it just a long string for the whole contract?
• could they have more than one contract template?
• either way, in the admin we could provide a way for them to create one or more contracts with "placeholder" fields for names and dates. I think that would be neat.

[smarbos]

• I think a long string for the contract is ok for starting.
• I think one contract template is ok to start. It could be defined in the main configuration of the app.
• I like the placeholder idea, but I think we should keep it simple for the start and add more features in the future.
  I think that using placeholders and multiple contracts templates will increase a lot the complexity of this funcionality.

Another think I would love to take care of is the legal aspect of this contract... Again, let's start with something simple and scale it later.