Consider adjusting some strategies to use passport-oauth2 instead of specific providers

[stefandesu]

passport-oauth2 is a core part of passport and therefore kept up-to-date, unlike some other strategies like passport-stack-exchange (see also #102). Since many sites use OAuth2, I think we should consider reducing our dependencies and use passport-oauth2 for those as well.

I'm particularly thinking about passport-orcid and passport-mediawiki-oauth.