Allow cancelling while submission in progress

[seadowg]

Acceptance

• [ ] Given I'm submitting forms (either with auto send or manually) When I click "Cancel" on the notification Then submission stops And any remaining forms are not sent

• [ ] Given I'm submitting forms (either with auto send or manually) When I open "Send Finalized Form" And I click cancel And any remaining forms are not sent Then submission stops And any remaining forms are not sent

Notes

We should make sure to cancel between media file uploads. Otherwise, it'd be possible to get stuck for a while sending a form with many media files. Like with downloading, the check for a cancelled "state" needs to be between forms and between media files.