Add a Mastodon Sharing Button

[macmanx2]

With Mastodon expanding, it might be nice to have a Mastodon share button on the posts.

Sharing to Mastodon is nowhere near as easy as sharing to non-federated platforms, but there are some options out there we could use (if the license is right) or be inspired by:

• https://github.com/Aly-ve/Mastodon-share-button
• https://github.com/sharetomastodon/sharetomastodon.github.io
• Sharing: add a Mastodon sharing button Automattic/jetpack#28694