Added optional urlfield in user profiles

I noticed one last little thing: Please add a space between `{{ _('Personal URL:') }}` and `<a href="{{userprofile.personal_url }}">`.

That's all that's left. Great job @tessie!  I will add a unit test when you're done (I won't ask you to do that yet, unless you'd really like to give it a try) and then it will be ready to merge.

Could you please squash all your commits into a single commit so I can merge it? I tried doing it myself but there was an issue because your branch already includes a merge commit.