[insiders] Add GraphQL mutations for organization user management and user invitation

[danh91]

Is your feature request related to a problem? Please describe.
There are know convenient ways of adding or inviting a user to join an organization

Describe the solution you'd like
A set of GraphQL queries and mutations to retrieve users and invitations from an organization, send and cancel invitations.
This will allow the integration of user management from the dashboard.

Describe alternatives you've considered
Currently, the only way to add users to an organization is by adding them manually from the admin console.