Added optional urlfield in user profiles

We need to be sure to use l10n whenever we are outputting a string to the user. In this case that would mean this line would change to:

`raise forms.ValidationError(_('The website is not reachable. Please enter a valid url.'))`

Note also that there is a space missing after the `.` in your message above, as well as a missing `.` at the end. 
Also we should be consistent with whether we use `URL` as you have in the label, or `url` as you have in this message.
Also, please use single quotes unless double quotes are necessary.