Publisher > New button should be deactivated for non-superusers

[rhigman]

thoth/thoth-api/src/graphql/model.rs

Line 1527 in a178ea8

// Only superusers can create new publishers - NewPublisher has no ID field

However, there are no checks in the app to prevent a non-superuser from attempting to create a new publisher. The user can click the "New" button, fill out all the fields and click "Save". Only at that point will they receive an "Invalid credentials" error (caused by the API check above).