Implement user fetching

[hmans]

When a new Pants::User is created, we should try and fetch their information from their website.

• [x] Rename Fetch#data to Fetch#document_data (since the user stuff will become Fetch#user_data)
• [x] Publish user JSON at /pants/user.json
• [x] Advertise the JSON URL as a link tag
• [x] Implement Pants::User.from_url:
  • [x] Extract user data from the user's URL
  • [x] Create or update the user record
  • [ ] Check if the URL has changed due to redirects