parameterize oauth2 provider

[scionaltera]

GitHub is hard coded in several places as the OAuth2 provider, but Spring Security supports several providers. It would be best to have a configuration option in mud.env so you can choose which of the supported providers you want to use, and to parameterize all the places in the code that currently refer to GitHub so you can switch without any code changes.

[scionaltera]

All that has to change for this is in the Freemarker templates there are a couple of hard coded URLs that reference "github". Those need to be able to change based on the OAuth flow that is actually in use.