Add a "Share via Mastodon" button

[LenzGr]

It's great that it's possible to share an article via various social media sites, e.g. Facebook, Twitter, LinkedIn and Telegram. As it's constantly growing in popularity, I would love to also have a "Share via Mastodon" button. There's a plugin available here: https://github.com/Aly-ve/Mastodon-share-button - adding it should be fairly straighforward (hence the "good first issue" label)