Created an oauth2 package which encapsulates all oauth2 providers,
utility functions, types, and interfaces. Previously some methods of the
Github provider were used as http.HandlerFuncs. These have now been
pulled into a concrete type called a JWTMux to implement other Oauth2
providers.
JWTMux has all of the functionality required to take a token from any
provider and store it as a JWT in a browser, and that is the extent of
its responsibilities. It implements the oauth2.Mux interface which would
potentially allow other strategies of oauth2 credential storage.
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