Add header to get public images. Cut source links.

[tedspare]

All thanks to @tommoor for this fix 🙌

Summary

• Use the new public-file-urls-expire-in header to fetch Linear markdown with public image URLs
• Remove image source links and their swapping logic

Test Plan

• Open a ticket with an image to the issue markdown via Linear. Ensure the image renders on GitHub.
• Edit the ticket with more images. Ensure new and old images render on GitHub.
• Repeat from GitHub. Ensure the image renders on Linear (there is some flash of "could not load image" before it re-uploads)

Related Issues

Closes #114

[tommoor]

I'm not sure what the behavior of the GitHub cache is, if we see that the images start to break after a day then you may want to increase this value.

[milospuac]

Great stuff, this will help me a ton 🙏🏻🫡