Add navigation for Edit Asset screen

[mikemartin]

Received some feedback from a client who was editing a lot of images:

"Currently setting the focal points for all the lifestyle images for prestige - is there a way to have it save in the background and move onto the next image without having to go back to the listing?"

Suggested fix:

• [ ] Add Save and Continue option, so users can save and navigate to next asset.
• [ ] Add keyboard navigation to go to next and previous assets