[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/numeric.go

@@ -0,0 +1,56 @@
+package qr
+
+import (
+	"errors"
+	"fmt"
+	"strconv"
+
+	"github.com/boombuler/barcode/utils"
+)
+
+func encodeNumeric(content string, ecl ErrorCorrectionLevel) (*utils.BitList, *versionInfo, error) {
+	contentBitCount := (len(content) / 3) * 10
+	switch len(content) % 3 {
+	case 1:
+		contentBitCount += 4
+	case 2:
+		contentBitCount += 7
+	}
+	vi := findSmallestVersionInfo(ecl, numericMode, contentBitCount)
+	if vi == nil {
+		return nil, nil, errors.New("To much data to encode")
+	}
+	res := new(utils.BitList)
+	res.AddBits(int(numericMode), 4)
+	res.AddBits(len(content), vi.charCountBits(numericMode))
+
+	for pos := 0; pos < len(content); pos += 3 {
+		var curStr string
+		if pos+3 <= len(content) {
+			curStr = content[pos : pos+3]
+		} else {
+			curStr = content[pos:]
+		}
+
+		i, err := strconv.Atoi(curStr)
+		if err != nil || i < 0 {
+			return nil, nil, fmt.Errorf("\"%s\" can not be encoded as %s", content, Numeric)
+		}
+		var bitCnt byte
+		switch len(curStr) % 3 {
+		case 0:
+			bitCnt = 10
+		case 1:
+			bitCnt = 4
+			break
+		case 2:
+			bitCnt = 7
+			break
+		}
+
+		res.AddBits(i, bitCnt)
+	}
+
+	addPaddingAndTerminator(res, vi)
+	return res, vi, nil
+}