[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/barcode.go

@@ -0,0 +1,42 @@
+package barcode
+
+import "image"
+
+const (
+	TypeAztec           = "Aztec"
+	TypeCodabar         = "Codabar"
+	TypeCode128         = "Code 128"
+	TypeCode39          = "Code 39"
+	TypeCode93          = "Code 93"
+	TypeDataMatrix      = "DataMatrix"
+	TypeEAN8            = "EAN 8"
+	TypeEAN13           = "EAN 13"
+	TypePDF             = "PDF417"
+	TypeQR              = "QR Code"
+	Type2of5            = "2 of 5"
+	Type2of5Interleaved = "2 of 5 (interleaved)"
+)
+
+// Contains some meta information about a barcode
+type Metadata struct {
+	// the name of the barcode kind
+	CodeKind string
+	// contains 1 for 1D barcodes or 2 for 2D barcodes
+	Dimensions byte
+}
+
+// a rendered and encoded barcode
+type Barcode interface {
+	image.Image
+	// returns some meta information about the barcode
+	Metadata() Metadata
+	// the data that was encoded in this barcode
+	Content() string
+}
+
+// Additional interface that some barcodes might implement to provide
+// the value of its checksum.
+type BarcodeIntCS interface {
+	Barcode
+	CheckSum() int
+}