Tooltip showing which sites support remote disk images

[Kwpolska]

The page for setting up a custom configuration requires disk images from "a supported site" and links to GitHub code search. Code search requires signing into a GitHub user account. It may be easier to link to the precise place in the codebase, like https://github.com/mihaip/infinite-mac/blob/fa69baec2f2ddb50d6cf7c271aeef823aecb738e/workers-site/cd-rom.ts#L38C5-L46C7 .

[mihaip]

It was somewhat intentional to use a code search (so that it would point to live values) as opposed to a URL with a commit hash embedded in it.

But all of that is a hack, what I should really do is pull the list of allowed sites into a shared constant and used it in both the worker and the UI.