"Warn before quitting" should not be required by default

On MacOS, when running CMD+Q, I would expect Element to close but I need to first confirm that I want to close the app. I believe the preference "Warn before quitting" should be off by default just like any other app where you don't lose anything when you close the app.

In order to prevent people from losing state, Element needs to implement `beforeunload` (or an equivalent) for non-recoverable state.