Inject util: Stop processing disconnected elements

[marcustyphoon]

Description

As noted in #1466 (comment) and previously addressed by #1501, it is possible for exceptions to be logged to the javascript console when we call certain injected functions on a target element that is no longer in the page by the time the injected function is executed.

As of #1514, it is usually fairly difficult for this to happen.

Nevertheless, it should be possible; the first time an injected function is executed it has to be imported asynchronously, for example.

This therefore uses a similar "just don't trigger the listener and resolve the promise" method to prevent any processing code from being run in this now-extremely-rare case. Fortunately, the new structure makes this a oneliner.

Testing steps

• Confirm basic functionality.

If one wants to observe the behavior this fixes:

• Load the extension without this PR. Enable at least one feature that calls timeline()
• Open the javascript console on Tumblr Patio (with at least one column that the feature processes).
• Click the Tumblr Patio button, triggering a refresh (redpop does not use soft navigate in this instance, so this will perform a full page load and full extension initialization).
• During the resulting post load, click "home" to navigate to the dashboard. This will—not always, but fairly reliably with the right click timing—log a bunch of errors to the console as unbury_timeline_object throws on every Patio post for every feature that uses it.
• Perform this same sequence of events with this PR and confirm there are no longer exceptions logged.

[marcustyphoon]

Oh, wait... I guess the target of an event has to be a Node, doesn't it. I forgot it defaulted to document.documentElement.