[BUG] Image source links protocol fix

As reported here: INC3474764

In notification generated for CERN News channels, which come from home.cern articles, images come with src="//cds.cern.ch/images/CERN-HOMEWEB-PHO-2023-067-1/file?size=large"

which causes outlook to fail to find images and freeze in certain cases.

Image links should then be changed to have https: so prevent further issues.

See Vincent's explanation from the ticket below:

Based on the image003.png, I believe that Outlook is interpreting this as a DFS link (\\...\) and that's probably why it's freezing and failing, as CDS probably doesn't offer this option...

On the "As far as browsers go, they are smart enough to take this reference and use https,", well that's only partially correct.
As far as I understand, `src=//domain/...` means "use the same protocol as the protocol used to fetch this resource", a.k.a. "Protocol-relative URLs": https://en.wikipedia.org/wiki/URL#prurl
This makes sense for sites which are pulled over from http or https, as the browser can use the corresponding protocol.
But for mails, which aren't fetched over http or https, it doesn't make sense...

So I don't think we should be configuring Outlook to do "proper action", since there isn't one.
The email itself should be fixed to have a proper URL.

I think the only things to be careful of are:
- If the site only supports http, then using https will break it.
- If the site supports https with a CERN certificate only, it might not load due to the certificate issue.
- Since https://outlook.office.com/mail/ is over https, if the image is loaded over http only, it might not load on some browser that block mixed content (even for image content)

Given that the world is more and more pushing for https, I would recommend to favour it, but it could break for poorly setup sites :/