prevent unicode-confusion in password by applying PRECIS, and username/email address by applying unicode NFC normalization

an é (e with accent) can also be written as e+\u0301. the first form is NFC,
the second NFD. when logging in, we transform usernames (email addresses) to
NFC. so both forms will be accepted. if a client is using NFD, they can log
in too.

for passwords, we apply the PRECIS "opaquestring", which (despite the name)
transforms the value too: unicode spaces are replaced with ascii spaces. the
string is also normalized to NFC. PRECIS may reject confusing passwords when
you set a password.

vendor/golang.org/x/text/width/kind_string.go

@@ -0,0 +1,28 @@
+// Code generated by "stringer -type=Kind"; DO NOT EDIT.
+
+package width
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[Neutral-0]
+	_ = x[EastAsianAmbiguous-1]
+	_ = x[EastAsianWide-2]
+	_ = x[EastAsianNarrow-3]
+	_ = x[EastAsianFullwidth-4]
+	_ = x[EastAsianHalfwidth-5]
+}
+
+const _Kind_name = "NeutralEastAsianAmbiguousEastAsianWideEastAsianNarrowEastAsianFullwidthEastAsianHalfwidth"
+
+var _Kind_index = [...]uint8{0, 7, 25, 38, 53, 71, 89}
+
+func (i Kind) String() string {
+	if i < 0 || i >= Kind(len(_Kind_index)-1) {
+		return "Kind(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _Kind_name[_Kind_index[i]:_Kind_index[i+1]]
+}