[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/README.md

@@ -0,0 +1,53 @@
+[![Join the chat at https://gitter.im/golang-barcode/Lobby](https://badges.gitter.im/golang-barcode/Lobby.svg)](https://gitter.im/golang-barcode/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+
+## Introduction ##
+
+This is a package for GO which can be used to create different types of barcodes.
+
+## Supported Barcode Types ##
+* 2 of 5
+* Aztec Code
+* Codabar
+* Code 128
+* Code 39
+* Code 93
+* Datamatrix
+* EAN 13
+* EAN 8
+* PDF 417
+* QR Code
+
+## Example ##
+
+This is a simple example on how to create a QR-Code and write it to a png-file
+```go
+package main
+
+import (
+	"image/png"
+	"os"
+
+	"github.com/boombuler/barcode"
+	"github.com/boombuler/barcode/qr"
+)
+
+func main() {
+	// Create the barcode
+	qrCode, _ := qr.Encode("Hello World", qr.M, qr.Auto)
+
+	// Scale the barcode to 200x200 pixels
+	qrCode, _ = barcode.Scale(qrCode, 200, 200)
+
+	// create the output file
+	file, _ := os.Create("qrcode.png")
+	defer file.Close()
+
+	// encode the barcode as png
+	png.Encode(file, qrCode)
+}
+```
+
+## Documentation ##
+See [GoDoc](https://godoc.org/github.com/boombuler/barcode)
+
+To create a barcode use the Encode function from one of the subpackages.