Refactor api_key model

[alrocar]

We are adding more and more logic to the AR model. We could extract logic for:

• Database helpers: grants, queries, etc.
• Grant helpers: Grant validation, etc.
• Actual model logic: Model validations, attributes, etc.

[gonzaloriestra]

Closing in favor of https://app.clubhouse.io/cartoteam/story/83461/refactor-api-key-model