sso_auth: add identity provider slug to url path and mux

[jphines]

Problem

As we move towards supporting multiple providers within sso-auth, we need to provide unique callback urls for each identity provider.

Solution

We add more provider slug prefixes to url paths and supply a default provider to support unique callback urls as well as preserve backward's compatibility

Notes

• Creates a new AuthenticatorMux struct to house the mux mechanism for multiple Authenticators
• Move initialization from main.go to NewAuthenticatorMux
• Make newStatsdClient public to call from initialization from main.go
• Add new Slug struct field to ProviderData to differentiate between providers.

[Jusshersmith]

Looks great - nice work @jphines! 🐙