UX: hop auth add

[myNameIsPatrick]

Description

It would be nice if hop auth add opened up the scimma-admin page rather than immediately prompt the user for username/password since it's not clear for a new user how to fill in this information.

[cnweaver]

My recollection is that opening a page in web browser can be difficult to do portably on Linux; https://github.com/slateci/minislate/blob/master/minislate#L577 was the best I could come up with on a previous project. There's also a philosophical question abut whether the client can know the right page to open. If we just declare hop.scimma.org the one blessed hopskotch instance, thet's easy. If, however, we want to admit other hopskotch instances operated by other groups, then there isn't just one definite address to use for this.