Revert "Remove the oauth_callback_url config option"

README.md

cmd/cashierd/main.go

example-server.conf

server/auth/github/github.go

server/auth/github/github_test.go

server/auth/gitlab/gitlab.go

server/auth/gitlab/gitlab_test.go

server/auth/google/google.go

server/auth/google/google_test.go

server/auth/provider.go

server/auth/provider_test.go

server/auth/testprovider/testprovider.go

server/config/config.go

server/config/config_test.go

server/config/testdata/test.config