normalize user.profile.website URL on update

When a user enters their website url, pullup will check for an http://
or https:// string at the beginning and add it if it doesn't exist.
This lets developers write clean <a href=user.profile.website> tags
later on down the road.
links filtered username on posts to website

When you view all posts by a certain username, point the
filteredUsername link to that user's website, if specified on the user's
account page. Othewise, just use a github link.