The spec for omniauth doesn't test signup for every provider, and it
doesn't test if it has tests for all available providers
Also, we no longer use secrets
* Add model test to check if the omniauth providers have changed
* Test signup using every provider
* Replace secretswith environment variables
The names and usernames don't follow a canonical pattern
And the emails point to a valid domain not owned by osem and not
reserved for illustration purposes
Normalize the users' names and usernames and change the domain of the
email addresses to example.com
Fix#1365
