Improve Export Applications modal

[torchiaf]

Summary

Fixes #301

Technical Notes

• Avoid close Export Applications modal when clicking outside
• Cancel download request on click close button

[richard-cox]

There's some strangeness after Canceling the modal and trying again

• The cancel side looks like it works fine
• Trying to download the image again though results in a failure and a bad error message in a banner

• The correct error should be presented to the user
• There should be a BE ticket to ensure that the job creating the exported image is cancelled if the http request is cancelled export of app image requests can be concurrent

With those two things addressed we're good to merge this

[torchiaf]

• The correct error should be presented to the user

I think the error is no more reproducible after refreshing my environment with latest main I reproduced the issue in the latest version of Epinio;
Not related to this issue: I realized we didn't handle esc button, just pushed a fix for this.

I just created a new issue to take note of the export error: epinio/epinio#2516 and pushed a change to handle the error messages on the UI.