Make mutations resolvers that are compatible with optimistic UI

[nenadjaja]

Resolvers need to return objects that we can use for Apollo caching in the UI. For example, addProject mutation should return a project object (once it's successful):

return  {
  id: "123",
  name: "One"
  description: "Blah blah"
  ...
}

If it's unsuccessful, it should return an error object that the UI will handle, instead of silently failing.

[nenadjaja]

This is implemented