storage: increase the number of bytes randomly generated for IDs

Because these values are used for OAuth2 codes, refresh tokens,
etc, they shouldn't be guessable. Increase the number of random
bytes from 64 to 128.

storage/storage.go

@@ -30,7 +30,7 @@ var encoding = base32.NewEncoding("abcdefghijklmnopqrstuvwxyz234567")
 
 // NewID returns a random string which can be used as an ID for objects.
 func NewID() string {
-	buff := make([]byte, 8) // 64 bit random ID.
+	buff := make([]byte, 16) // 128 bit random ID.
 	if _, err := io.ReadFull(rand.Reader, buff); err != nil {
 		panic(err)
 	}