all: refactor handler logic

- all: prefer `FormValues` to `ParseForm` with subsequent `Form.Get`s
- all: refactor authentication stack to be checked by middleware, and accessible via request context.
- all: replace http.ServeMux with gorilla/mux’s router
- all: replace custom CSRF checks with gorilla/csrf middleware
- authenticate: extract callback path as constant.
- internal/config: implement stringer interface for policy
- internal/cryptutil: add helper func `NewBase64Key`
- internal/cryptutil: rename `GenerateKey` to `NewKey`
- internal/cryptutil: rename `GenerateRandomString` to `NewRandomStringN`
- internal/middleware: removed alice in favor of gorilla/mux
- internal/sessions: remove unused `ValidateRedirectURI` and `ValidateClientSecret`
- internal/sessions: replace custom CSRF with gorilla/csrf fork that supports custom handler protection
- internal/urlutil: add `SignedRedirectURL` to create hmac'd URLs
- internal/urlutil: add `ValidateURL` helper to parse URL options
- internal/urlutil: add `GetAbsoluteURL` which takes a request and returns its absolute URL.
- proxy: remove holdover state verification checks; we no longer are setting sessions in any proxy routes so we don’t need them.
- proxy: replace un-named http.ServeMux with named domain routes.

Signed-off-by: Bobby DeSimone <bobbydesimone@gmail.com>

authorize/authorize.go

@@ -14,7 +14,7 @@ import (
 func ValidateOptions(o config.Options) error {
 	decoded, err := base64.StdEncoding.DecodeString(o.SharedKey)
 	if err != nil {
-		return fmt.Errorf("authorize: `SHARED_SECRET` setting is invalid base64: %v", err)
+		return fmt.Errorf("authorize: `SHARED_SECRET` malformed base64: %v", err)
 	}
 	if len(decoded) != 32 {
 		return fmt.Errorf("authorize: `SHARED_SECRET` want 32 but got %d bytes", len(decoded))