Refactor LinkField around FormSchema

[maxime-rainville]

As a CMS Squad, we want the LinkField to use best practices to manage its communication with the backend so it's easy to maintain in the long run.

Acceptance criteria

• Only the Link ID is communicated to the front end (rather than a big JSON blob)
• Initial Link Title and Description is fetched via a scaffolded GraphQL schema.
• The form schema in the Link Modal is retrieved via the ID of the link being edited.
• When the Link modal is submitted, the link is created/updated on the spot.
• LinkField is updated to remove the logic to handle scenarios where the type of a link is changed.

Exclude

• Form schema validation

PRs

• NEW LinkFieldController to handle FormSchema #108

[maxime-rainville]

We initially wanted to block this card on "Standardised how we create API/FormSchema endpoint in the CMS". We decided to reverse the block and use this card as a POC. We'll learn lessons from this card and reimplement it later on to use the new standard.

[maxime-rainville]

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