Feature request: mastodon support in social links

[acut3]

Description

With Mastodon gaining popularity, it would be great if support could be added.

This is especially important because Mastodon has this functionality where it can automatically "verify" profile links if they point back to your Mastodon profile.

Proposed change

Add the following block to _includes/author-links.html:

    {%- if _author.mastodon -%}
      <li title="{{ _locale_string_follow | replace: '[NAME]', 'Mastodon' }}">
        <a class="button button--circle mastodon-button" itemprop="sameAs" rel="me" href="https://{{ _author.mastodon }}" target="_blank">
          <div class="icon">{%- include svg/icon/social/mastodon.svg -%}</div>
        </a>
      </li>
    {%- endif -%}

Notice the rel="me" attribute, which is required for the account verification process.

In the _config.yml file, author.mastodon would be set to something like mastodon.social/@handle. The server's name (here, mastodon.social) has to be included.

The hard part is making a round Mastodon icon and the associated styles (which is what's preventing me from submitting a PR).