Allow viewports to specify a custom user-agent

This addresses https://github.com/trotzig/diffux/issues/62

I started taking snapshots of apartmentlist.com and realized that they
are doing user-agent sniffing. To have snapshots look the way they do on
peoples phones etc, I added the ability to specify a user-agent string
on a viewport.

Viewports didn't have an edit page before, so I added one and linked to
it from the list of viewports a project page has (I moved them down from
the header to the info-table).