Favicon tidy-ups

[zerolab]

Based on https://evilmartians.com/chronicles/how-to-favicon-in-2021-six-files-that-fit-most-needs

To-Do:

• finish the svg favicon. Could use some help here @nicklee (bonus points: use @media (prefers-color-scheme: dark) for dark mode styling in the same svg)
• export a 512x512 variant to wagtailio/static/img/favicons/android-icon-512x512.png (currently it is a copy of the 192x192 one)

Nice to have - revised favicon. currently is using the old #43b1b0 background. Could probably move to the #2e1f5e (as the primary colour in Wagtail 4) or the grey one as in the default sharing image

[zerolab]

@vossisboss would be cool if you could stir up some resources for this, if at all possible
tl;dr -- need a nice SVG logo variant, ideally with a dark scheme version

[vossisboss]

@zerolab So just to confirm, we need two variants (light and dark) on a new favicon that can be exported to dimensions of 512 X512?

If that's the case, I might be able to get Ben to slip it in next week when he rotates into his Wagtail design time.

[zerolab]

@vossisboss that's right

[nicklee]

Images added, including a dark mode. The Favicon is now a closer crop of the wagtail's head.

[nicklee]

I've just merged this into the new dark mode MR here: #391

Semi linked now we have a dark mode favicon :)