Returns a single styleguide that user has access.

User is considered eligible to access a styleguide if any of the following conditions is met:

• User is a member of the styleguide
• User is a member of the organization that the styleguide belongs to
• User is a member of a project which is linked to the styleguide (using linked_project parameter)
• User is a member of a project and the styleguide is an ancestor of the project's linked styleguide (using linked_project parameter)
• User is a member of a styleguide that the styleguide is one of its ancestors (using linked_styleguide parameter)