scripts: Handle redirects in image URLs

[garrett]

GitHub changed the way they handle images; their URLs might not have an extension now. For the time being, we should assume the images without extensions are PNG files.

We will probably want to download whatever they are and run some identification on them in the future (like the file command).

I also switched the string extension and string concat to use native Ruby File.* commands.

[jelly]

This does not seem to work for me:

Re-added release-note for this image cockpit-project/cockpit#17796 which works but now get's an extra .. the other image is empty.

[jelle@t14s][~/projects/cockpit-project.github.io]%file images/299-networking-add-support-for-wireguard-2.png
images/299-networking-add-support-for-wireguard-2.png: empty
[jelle@t14s][~/projects/cockpit-project.github.io]%file images/299-storage-set-up-a-system-to-use-nbde..png
images/299-storage-set-up-a-system-to-use-nbde..png: PNG image data, 853 x 313, 8-bit/color RGBA, non-interlaced

[jelly]

This does not seem to be fixable by this script, we can't get the images even if we hack the url:

Working old url:

https://user-images.githubusercontent.com/3228183/211269003-613885c7-6a11-4416-b69d-6435560ec49a.png

New url:

https://github.com/cockpit-project/cockpit/assets/200109/bb33f362-a4ac-470e-a9b9-62ee5c3d3116

Modify into user-images url just fails so ugh not much we can do.

curl https://user-images.githubusercontent.com/200109/bb33f362-a4ac-470e-a9b9-62ee5c3d3116.png
<?xml version="1.0" encoding="UTF-8"?>
<Error><Code>AccessDenied</Code><Message>Access Denied</Message><RequestId>TRQ7PN4ARBTBNPFN</RequestId><HostId>x8JWBK0ueMfd6TzZK5GVAi4XiYSUGkr/mtve1V/z120GXG5ACa7z32fGDLVkL0B5PTnumegDPiY=</HostId></Error>%

[garrett]

Addressed your issues and integrated your suggestions @ #657, but added multiple redirect support (recursive calling based on response) and error handling.

If this is merged, it:

closes #657

[jelly]

Would have liked to have a comment about the new urls which we need to handle now but let's get it merged. 👍