Mobile firefox is not using favicon

[rgaudin]

As can be seen in #18's screenshot (Firefox on iOS/iPadOS), the favicon is not displayed and uses a browser-default with the first name of the Title (D).

I cannot reproduce on any other browser: FF, iOS, chrome all OK on desktop. On iOS, Safari and DuckDuckGo are OK but FF is not.

On Chrome for iOS favicons seems barely used (only in bookmarks?) and there it doesn't seem to pick it up but it doesn't picks up Github's so I wouldn't worry much.

Note that favicon is specified in a complicated way in an attempt to have it working properly on both light and dark modes.