cross-link owner pico (Google vs GitHub vs SSI)

[b1conrad]

[b1conrad]

When an owner has signed up with Google, we create an owner pico (using their Google id number as the pico name). We would like to give an option to let the owner login with their GitHub account instead and get access to the original owner pico.

Even more important, once we have signup/login via SSI (issue #7), we'd like owners with older (Google/GitHub) owner picos to be able to link their new SSI with their old owner pico.