Add Mastodon social SVG

[maclarel]

This adds the Masotdon icon in SVG format:

Sourced from https://upload.wikimedia.org/wikipedia/commons/4/48/Mastodon_Logotype_%28Simple%29.svg, with minor tweaks to color/fill/etc... to align with the rest of the icons. It looks a bit odd on a white background with stroke-width="2" but I kept this to align with all other icons. A stroke width of 16 looks far better IMO:

Not quite perfectly aligned with the rest as viewBox is inflated and I'm not sure how to rescale everything, but hopefully this can either be accepted or riffed on further by maintainers.

Thanks for the consideration!