Add social media and author information to default templates

[preaction]

It would be nice if the user could configure a bunch of information about themselves easily, without having to alter the default templates. These are things that a lot of people will likely want when building a personal website or a project website, like:

Plain content

• About the author/project
• Author portrait
• Contact information
  • E-mail
  • Phone
  • IM handles?
  • Mailing address
• CPAN author ID
• Patreon/Gratipay?

Social Media

• Twitter handle
  • Set a flag to add a Twitter widget too?
• Facebook page name/URL
  • Set a flag to add a Facebook widget too?
• Google+ name
  • Set a flag to add a G+ widget too?
• Github username
• Pinterest
• Reddit
• YCombinator Hacker News?
• Imgur
• Meetup group
  • Set a flag to add a Meetup widget too?
• Youtube?
• Flickr
• Instagram

Site information

• Content licensing
• Related sites / Blogroll (this should be a nav)

These things should be placed into the sidebar and/or site footer of every page.

[preaction]

This very much complicates the default themes, and should be left out of the "simple" themes (#238).

[preaction]

In order to do this, we need to move the sidebar to the layout. In order to do that, we need to be able to add things to the sidebar from the page template and the document content. Added as #416