Prevent event propogation if element is disabled when event chain begins.

The existing UJS event behavior relies on browsers not sending events for
various events when an element is disabled. For example, imagine the following:

    <button type="submit" disabled="disabled">Click me</button>

The above button is disabled, so browsers will not trigger a click event and
all UJS behavior is prevented. However, imagine a button like this:

    <button type="submit" disabled="disabled"><strong>Click me</strong></button>

The above is treated differently by browsers such as Chrome/Safari. These
browsers do not consider the strong tag to be disabled, and will trigger click
events. UJS has logic to walk up the DOM to find an associated element subject
to UJS behavior. But, this logic does not take into account the disabled
status of the element.

I originally thought we could simply change the selectors used to match
elements to ignore disabled elements. However, UJS disables some elements as
part of the event chain. So, an element might match early in the chain and
then fail to match later. Instead of changing the selectors I added a callback
to the chain that calls `stopEverything` if an element is disabled when the
event chain begins.

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