Modifier keys ignored for form submissions

[mburtscher]

For clicks on regular hyperlinks, Turbo does notice when a modifier key like CMD (new tab) or SHIFT (new window) is pressed and will skip Turbo navigation in favor of a native navigation action. This leads to a consistent behavior of Turbo powered apps.

However for form submissions, these modifier keys are ignored and Turbo navigation happens even if modifier keys are used.

An implementation like LinkClickObserver.clickEventIsSignificant must be added to the FormSubmitObserver as well.