Recompilation speedup: create WrapperAuthenticator and simplify Authenticator behaviour

#2651 (closed)

Web.Auth.Authenticator has a similar problem as !3440 (merged), !3450 (merged) - it's both a behaviour and a config Wrapper module, creating compile-time cycles.

I broke it up into 3 separate modules:

• Web.Auth.Authenticator - now strictly a behaviour, only contains @callback's
• Web.Auth.WrapperAuthenticator - an Authenticator implementation that dynamically selects the auth module based on config
• Web.Auth.Helpers - contains helper functions originally from Web.Auth.Authenticator

Now there's one less cycle (a big one) and the code is more idiomatic: https://www.diffchecker.com/534XPg3w