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/cryptutil/encrypt.go

@@ -13,20 +13,27 @@ import (
 	"golang.org/x/crypto/chacha20poly1305"
 )
 
+// DefaultKeySize is the default key size in bytes.
 const DefaultKeySize = 32
 
-// GenerateKey generates a random 32-byte key.
+// NewKey generates a random 32-byte key.
 //
 // Panics if source of randomness fails.
-func GenerateKey() []byte {
+func NewKey() []byte {
 	return randomBytes(DefaultKeySize)
 }
 
-// GenerateRandomString returns base64 encoded securely generated random string
-// of a given set of bytes.
+// NewBase64Key generates a random base64 encoded 32-byte key.
 //
 // Panics if source of randomness fails.
-func GenerateRandomString(c int) string {
+func NewBase64Key() string {
+	return NewRandomStringN(DefaultKeySize)
+}
+
+// NewRandomStringN returns base64 encoded random string of a given num of bytes.
+//
+// Panics if source of randomness fails.
+func NewRandomStringN(c int) string {
 	return base64.StdEncoding.EncodeToString(randomBytes(c))
 }