Revisions: add a copy button

On the Revisions tab in the Layouts or Pages interfaces, you can see the code editor displaying templates and Elixir code but there's no shortcut to copy such content. We want to add a copy button next to those elements using the [beacon_admin:clipcopy event](https://github.com/BeaconCMS/beacon_live_admin/blob/0af5d7835843d873313b4eb5cd245e6539ddfae3/assets/js/beacon_live_admin.js#L21). The Media Library has implemented the same feature, that's a good example to check out.