Add index to created_at

When the `telescope_entries` table gets large, `telescope:prune` will fail miserably, actually bringing an AWS db instance to its knees. This is because `telescope:prune` is using `created_at` in a where clause when there's no index on that column.

src/Storage/migrations/2018_08_08_100000_create_telescope_entries_table.php

@@ -48,7 +48,7 @@ public function up()
             $table->boolean('should_display_on_index')->default(true);
             $table->string('type', 20);
             $table->longText('content');
-            $table->dateTime('created_at')->nullable();
+            $table->dateTime('created_at')->nullable()->index();
 
             $table->unique('uuid');
             $table->index('batch_id');