Admin panel should warn to save before navigating to a new tab

When customizing the site on a Hubs Cloud instance, there is no warning that tabbing to a different section will clear the input of the current tab. We should add a warning to this so that users know to save before changing other settings

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/HUBS-1033)

Start at the admin panel's settings tab navigation and reproduce switching sections with unsaved input. Review the existing save behavior and the tab-change flow; done when users receive a warning to save before changing sections so current-tab input is not silently cleared.

Written by the indexing model from the issue text.