From the Resources page, add resources to an already existing diagram

[svozza]

Is your feature request related to a problem? Please describe.
It would help as a workaround to #351, allowing users to add resources to a diagram in smaller batches.

Describe the feature you'd like to see implemented
Currently you can only add multiple resources to a diagram once: at creation time. It would be nice if resources could be added to and existing diagram too.

Describe the value this feature will add to AWS Perspective
Tell us how this feature might improve AWS Perspective.

Describe alternatives you've considered
The only alternative is adding them one by one using the search bar on the Diagram page.