Revert "remove thumbnail cache"

This reverts commit e0db4931f1.

app/util/state.ts

@@ -15,7 +15,3 @@ export function mergeById<T extends { [id: string]: unknown }>(object: T, source
 export function mapById<T>(object: ById<T>, ids: string[]): T[] {
   return ids.map(id => object[id]).filter(a => a !== undefined)
 }
-
-export function mapId(entities: { id: string }[]): string[] {
-  return entities.map(e => e.id)
-}