[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/qr/unicode.go

@@ -0,0 +1,27 @@
+package qr
+
+import (
+	"errors"
+
+	"github.com/boombuler/barcode/utils"
+)
+
+func encodeUnicode(content string, ecl ErrorCorrectionLevel) (*utils.BitList, *versionInfo, error) {
+	data := []byte(content)
+
+	vi := findSmallestVersionInfo(ecl, byteMode, len(data)*8)
+	if vi == nil {
+		return nil, nil, errors.New("To much data to encode")
+	}
+
+	// It's not correct to add the unicode bytes to the result directly but most readers can't handle the
+	// required ECI header...
+	res := new(utils.BitList)
+	res.AddBits(int(byteMode), 4)
+	res.AddBits(len(content), vi.charCountBits(byteMode))
+	for _, b := range data {
+		res.AddByte(b)
+	}
+	addPaddingAndTerminator(res, vi)
+	return res, vi, nil
+}