Enable IndieWeb support

IndieWeb is a trending project, focused mainly on allowing website owners to own and control their content, and also stay connected with engagement on them across services. All it needs is availability of some key classes on elements. This does not need any action on user's part, at the same time enabling below.

• Set up web sign-in via rel-me
• Add info about owner via h-card
• Add microformats to content via h-entry
• Allow configuring webmention endpoint