refactor: fix type error with flattening arrays

src/common.ts

@@ -51,8 +51,7 @@ export async function getProjects(api: Api, groups: number[]): Promise<Project[]
  * @param arr Flattened array
  */
 export function flatten2dArray<T>(arr: T[][]): T[] {
-  // TODO: fix this
-  return [].concat.apply([], arr as any);
+  return ([] as T[]).concat(...arr);
 }
 
 /**