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/config/policy_test.go

@@ -1,4 +1,4 @@
-package config // import "github.com/pomerium/pomerium/internal/config"
+package config
 
 import (
 	"testing"
@@ -44,3 +44,28 @@ func Test_Validate(t *testing.T) {
 		})
 	}
 }
+
+func TestPolicy_String(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name string
+		From string
+		To   string
+		want string
+	}{
+		{"good", "https://pomerium.io", "https://localhost", "https://pomerium.io → https://localhost"},
+		{"failed to validate", "https://pomerium.io", "localhost", "https://pomerium.io → localhost"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			p := &Policy{
+				From: tt.From,
+				To:   tt.To,
+			}
+			p.Validate()
+			if got := p.String(); got != tt.want {
+				t.Errorf("Policy.String() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}