Add related mutations (#43)

src/schema.graphql

@@ -263,7 +263,7 @@ type Product {
   ref2: String
   ref3: String
   related: [Product]
-  wallpaperTypes: [ProductWallpaperType]
+  wallpaperTypes: [ProductWallpaperType]!
   fields: ProductFields
   orientation: Orientation
   """
@@ -454,5 +454,9 @@ type Mutation {
     productId: Int!
     wallpaperTypes: [ProductWallpaperType]!
   ): Product
+
+  addRelatedProducts(productId: Int!, articleNumbers: [String]!): Product
+  removeRelatedProducts(productId: Int!, relatedProductIds: [Int]!): Product
+
   addKeyword(name: String!, type: KeywordType): Keyword
 }