Use quotes for circular field definitions

What does this MR do and why?

While converting to the new GraphQL interpreter, ran across a circular dependency with Types::GroupType, which would give

An error occurred while loading ./spec/graphql/resolvers/user_resolver_spec.rb.
Failure/Error: base.class_eval(&@_prepended_block)

NameError:
  uninitialized constant Types::EpicType::GroupType
  Did you mean?  Types::GroupType
                 GroupTree

Solution is to quote the type, from

field :group, Types::GroupType

to

field :group, 'Types::GroupType'

See https://github.com/rmosolgo/graphql-ruby/issues/1929#issuecomment-434037910

Related to #210556 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.