Make `viewport_widths` and `url_addresses` fields required

This doesn't fully resolve issue #90 and #91, but it takes care of some
of it.  Before this change, you could create a project without any URL
addresses and/or viewport widths. This is still the case, even after
this change (we parse these fields on the server and ignore anything
that's unparseable). Making the fields required fixes the problem where
you could save a project with these fields blank.
Add default widths and URL addresse to project form

By having an example of what you can type in this field, I think people
are going to more easily understand what format they need to use.

I went with mobile (320) and desktop (1200) for the widths field and
added 'http://www.example.com/' in the URLs field.