Add Join page

This adds a Join page where people new to the site can join up. The
joining process only requires a username and password, we don't do any
email validation or other checks.

After joining a user will be automatically logged in.
Style the front page

This adds semantic-ui css (imported via bower) to the application,
providing some nice formatting defaults.

A background picture has been added, the picture is of a clock, of
course.

Overall, this change moves the web page actually some way to looking
like a real website.