[feature] add TOTP two-factor authentication (2FA) (#3960)

* [feature] add TOTP two-factor authentication (2FA)

* use byteutil.S2B to avoid allocations when comparing + generating password hashes

* don't bother with string conversion  for consts

* use io.ReadFull

* use MustGenerateSecret for backup codes

* rename util functions

vendor/github.com/boombuler/barcode/utils/runeint.go

@@ -0,0 +1,19 @@
+package utils
+
+// RuneToInt converts a rune between '0' and '9' to an integer between 0 and 9
+// If the rune is outside of this range -1 is returned.
+func RuneToInt(r rune) int {
+	if r >= '0' && r <= '9' {
+		return int(r - '0')
+	}
+	return -1
+}
+
+// IntToRune converts a digit 0 - 9 to the rune '0' - '9'. If the given int is outside
+// of this range 'F' is returned!
+func IntToRune(i int) rune {
+	if i >= 0 && i <= 9 {
+		return rune(i + '0')
+	}
+	return 'F'
+}