Add a social pointer to Mastodon (specifically, our new official home at Fosstodon). This also serves as a verification link for Fosstodon, and the `rel="me"` is required, which is why it's being handled out-of-band from the other links; if we're comfortable always using `rel="me"` for social links, we could unify this.
Add a social pointer to Mastodon (specifically, our new official home at Fosstodon). This also serves as a verification link for Fosstodon, and the `rel="me"` is required, which is why it's being handled out-of-band from the other links; if we're comfortable always using `rel="me"` for social links, we could unify this.

Fixes #403.

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