Rewrite addAccountToGroup to not call through to Meteor method

[aldeed]

• Rewrite addAccountToGroup GraphQL resolver to not use context.callMeteorMethod.
  • https://docs.reactioncommerce.com/docs/graphql-create-mutation
  • https://docs.reactioncommerce.com/docs/graphql-create-mutation#copying-code-from-an-existing-meteor-method
• If nothing else calls the Meteor method, remove it.
• If this was the last use of context.callMeteorMethod anywhere, remove it from context.
• Write at least one Jest integration test for the mutation, if there isn't already one.

[aldeed]

Done in #5431