quickstart no longer available on website: change URL in extension

[jeffbl]

When you first install the IMAGE extension, it pops up a dialog pointing to a quickstart. This no longer exists. The extension should be updated to use the link:

https://image.a11y.mcgill.ca/tutorial/

[jeffbl]

Alternatively, we could redirect that URL to avoid having to rebuild and deploy the extension, but the right fix is to update the URL.