Notify the user when they are in a custom security level (desktop)

Split from #43819 for desktop.

With the recent changes to Security Level, I was thinking that currently communicating that the user is in a (bad) custom state on desktop requires the user to:

1. Have the security level button in the toolbar, which they could have customised. And in in Mullvad Browser it is not even present by default.
2. Notice the yellow dot (visual only), or read the tooltip for the button (mouse user hover, or a keyboard screen reader user would have to navigate and focus the button).

So, to make this state more noticeable and accessible, I think we should show a notification in the usual notification area (same place as the language notification when you first open the browser). We should show this any time the user switches from a non-custom state to a custom state. We should also show it at startup until the user explicitly dismisses the notification.

I think we can re-use the existing custom security level body text. And add a "Settings..." button.

@donuts, @felicia are you ok with this?