GraphQL mutations return a "Schema is not configured for mutations" error.

Steps to reproduce:

1. Access GraphiQL interface
2. Perform the following request:

mutation createFolder {
  createFolder(
    language: eng_GB
    parentLocationId: 2
    input: {
      name: "New Folder"
    }
  ) {
    id
  }
}

Expected result: A folder is created correctly

Actual result: The following error response is displayed:

{
  "errors": [
    {
      "message": "Schema is not configured for mutations.",
      "extensions": {
        "category": "graphql"
      },
      "locations": [
        {
          "line": 2,
          "column": 1
        }
      ]
    }
  ]
}

This applies for all mutations described in https://doc.ibexa.co/en/master/api/graphql_operations/

Note: Other requests, such as querying content, work correctly when changes from https://github.com/ezsystems/ezplatform-graphql/pull/90 are included (not documented yet)