Refactor labels management from HAML to Vue - Add `subscribed` argument to GraphQL query

Proposal

Add subscribed argument to app/graphql/resolvers/labels_resolver.rb. That argument should then be passed to the app/finders/labels_finder.rb so only subscribed labels are returned.

argument :subscribed, GraphQL::Types::Boolean,
          required: false,
          description: 'Only return labels current user is subscribed to.',
          default_value: false