Refactor external oauth providers, add GitHub, use osincli.Client

Refactor auth mux wiring

Code review

Remove custom String() functions, dump config in verbose logging only

Move token auth failure logging into failure handler

Validate state, redirect inside success handler, add success handler chain

Code review 2: godoc, require error handlers and use consistently, fix closure bug
bump(github.com/RangelReale/osincli): 23618ea0fc3faa3f43954ce8ff48e31f5c784212