Improve users list.

    - Add action to view the user's detail;
    - Add href='mailto...' to the email;
    - Make entire <td> cell clickable (display: block on 'a' tag).
Show all details of one single user.

User.show_user() turns all NULL fields into '&nbsp;' to avoid
empty tags in the html to break the layout/css. Perhaps I'll
have to come up with a better solution in the future.
Merge branch 'show_user'
Make layout use total width available.

It seems to be working fine and scaling to the browser
viewport's width. More testing is needed, though.
Rewrite markup for valid html5.

    - Replace & with &amp;
    - Replace spaces with %20.