Adds a cache forever tag to robohashes

app/src/main/java/com/vitorpamplona/amethyst/ui/components/Robohash.kt

@@ -95,6 +95,7 @@ object Robohash {
             .Builder(context)
             .data("robohash:$message")
             .fetcherFactory(HashImageFetcher.Factory)
+            .addHeader("Cache-Control", "max-age=31536000")
             .build()
     }
 }