Add hotkey to submit annotation

[leoll2]

Suggestion from @BrittvanAkker:

To make annotation even more user-friendly, having a hotkey for submitting annotated/reviewed images is very handy to avoid back and forth between the labels and the Submit button. From user feedback we learned that this hotkey is critical for their user experience. Expectation is that e.g. “s” (and change the hotkey for auto-segmentation) or “ctrl+s” would serve as a hotkey.