Add a .once() guard on File module's AJAX uploads to prevent double-uploading

[quicksketch]

Describe your issue or idea

This is a related issue to the double-binding problem we have with AJAX events: #3210

That PR will solve the double-throbber problem in the most common case, but other AJAX events can result in the same double-binding problem for File module.

Steps to reproduce (if reporting a bug)

• Add a post at node/add/post
• For testing, open the developer tools in your browser and enable connection throttling down to "3G" in the network tab.
• Upload an image (at least 2MB).
• Remove the image.
• Upload an image again.

Actual behavior (if reporting a bug)

• During the second upload you'll see the throbber displayed twice.

Expected behavior (if reporting a bug)

• During the second upload only a single throbber should be shown.

This is actually worse than it looks because the second image is actually uploaded twice, doubling the amount of time it takes to upload on a slow connection.

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PR by @quicksketch: backdrop/backdrop#2252

[alexfinnarn]

I can confirm that the attached PR fixes the double throbber issue; however, the tests are failing. I can't see which tests are failing.

[klonos]

I also confirm that the issue with the double throbber is fixed in the PR sandbox. Thanks for filing this ticket and for providing a fix @quicksketch 👍...this has annoyed me oh so many times (my OCD again 😜).

The failing tests are pointing to a missing /core/misc/zen-ci/init_test.sh file ...which if not mistaken has something to do with the recent work @Gormartsen and @serundeputy have been doing in order to allow zenCI to run phpcs (#3213). ...I think.

[serundeputy]

@klonos nah; nothing from #3213 is merged yet. It was just a hiccup with the test suite/runners ... I restarted and it is happier now: https://zen.ci/backdrop/backdrop/test/test-php53-backdrop_backdrop_2252-c9c7d06

[klonos]

Thanks @serundeputy 👍 ...the php70 test has gone AWOL though.

[serundeputy]

@klonos i turned it on and off again and everyone is happy :) backdrop/backdrop#2252

[klonos]

@serundeputy don't be fooled ...did you actually expand the "All checks have passed" fieldset and then check the "details" links for php53 and php70? ...as I said, AWOL:

...this bug with zen CI has been happening for a while when you close and reopen a PR:

[serundeputy]

this is an expanded one: https://zen.ci/backdrop/backdrop/deploy/deploy-backdrop_backdrop_2252-c9c7d06-2 but i think we are getting off track on this issue; if there are issues with testing you wish to sort out file them and scope them.

[quicksketch]

Super, I've merged backdrop/backdrop#2252 into 1.x and 1.10.x. Thank @alexfinnarn and @klonos for reviewing!