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>

internal/httputil/router.go

@@ -0,0 +1,19 @@
+package httputil // import "github.com/pomerium/pomerium/internal/httputil"
+
+import (
+	"net/http"
+
+	"github.com/gorilla/mux"
+	"github.com/pomerium/csrf"
+)
+
+// NewRouter returns a new router instance.
+func NewRouter() *mux.Router {
+	return mux.NewRouter()
+}
+
+// CSRFFailureHandler sets a HTTP 403 Forbidden status and writes the
+// CSRF failure reason to the response.
+func CSRFFailureHandler(w http.ResponseWriter, r *http.Request) {
+	ErrorResponse(w, r, Error("CSRF Failure", http.StatusForbidden, csrf.FailureReason(r)))
+}