GraphQL mutation for creating a new organization

We need a GraphQL mutation to create a new organization. Will be used for #417878 (closed)

This issue represents that bare minimum needed to create an Organization.

Avatar support is added through #422418 (closed) and #422597 (closed).

Designs

For context this API will be used for this UI

#410343 (closed)

https://gitlab.com/gitlab-org/gitlab/uploads/c521fdd985c14775815a680762527fca/New_organization_form.png

Requirements

Inputs

• name
• path