Index Sets page enable/disable stats selection should be persisted

## What?

#15155 introduced performance improvements to the Index Sets page, including a toggle for stats that defaults to disabled. If a user clicks the button to toggle stats the decision should be persisted so users who _do_ want to see stats don't have to click the `Enable stats` button each time they visit the page.

## Why?

One less click whenever I visit the Index Sets page.

## Your Environment

* Graylog Version: 5.2.0