Improve the UX of the maximized window warning

(spinning off !499 (merged))

The maximization notification has been broken for a while, so, while fixing it, we thought of a few improvements.

1. Add an additional button to enable letterboxing again
2. Move it to base-browser, since letterboxing is enabled also there.
3. Trigger it for any custom size, not only for maximization

While moving it to base-browser, we should add some migration for the counter preference (it refers torbutton), and we might try to use Fluent for it (I've already done it for the language notification: it was simple enough that Weblate worked with it).