Migrating code to JSON data based atoms

• Replace atom.data with atom in cell tests.
• Replace :redactor with :richtext in STRUCTURE definitions
• Move attachments keys (cover/images/document/documents) to a new ATTACHMENTS constant
• Move model key to a new ASSOCIATIONS constant

# before
STRUCTURE = {
  cover: true,
  title: :string,
  content: :redactor,
  model: %w[Folio::Page]
}

becomes

# after
ATTACHMENTS = %i[
  cover
]

STRUCTURE = {
  title: :string,
  content: :richtext,
}

ASSOCIATIONS = {
  model: %w[Folio::Page],
}