fix(server): Fix use of deprecated api for file uploads

See https://github.com/apollographql/apollo-server/issues/2105#issuecomment-448424473

server/src/service/services/asset.service.ts

@@ -45,7 +45,8 @@ export class AssetService {
      * Create an Asset based on a file uploaded via the GraphQL API.
      */
     async create(input: CreateAssetInput): Promise<Asset> {
-        const { stream, filename, mimetype } = await input.file;
+        const { createReadStream, filename, mimetype } = await input.file;
+        const stream = createReadStream();
         return this.createAssetInternal(stream, filename, mimetype);
     }