Add handling for Mastodon

[freakboy3742]

What is the problem or limitation you are having?

Following the Twitocalypse, BeeWare dropped its usage of Twitter. We've just opened a Mastodon account; we need to update the BeeWare website to reflect this.

Describe the solution you'd like

• The data model for core team members should allow them to associate Mastodon accounts, and this detail should be included on team member pages.
• The project homepage should include a rel="me" link for each team member with a Mastodon account so that the core team can link their association with beeware.org as a verified link. Individual team member pages should also include a rel="me" link. The link on the homepage should be invisible (i.e., display:none); the link on the team member page should be visible, with an appropriate icon.
• Blog posts should allow for mastodon links as the author. Ideally this would be a "if mastodon, link to mastodon; elif twitter, link to twitter; else no link".

Describe alternatives you've considered

No real alternative other than continuing to use twitter.

Additional context

No response