Use mattermost v5 module (#1192)

vendor/github.com/mattermost/go-i18n/i18n/language/plural.go

@@ -0,0 +1,40 @@
+package language
+
+import (
+	"fmt"
+)
+
+// Plural represents a language pluralization form as defined here:
+// http://cldr.unicode.org/index/cldr-spec/plural-rules
+type Plural string
+
+// All defined plural categories.
+const (
+	Invalid Plural = "invalid"
+	Zero           = "zero"
+	One            = "one"
+	Two            = "two"
+	Few            = "few"
+	Many           = "many"
+	Other          = "other"
+)
+
+// NewPlural returns src as a Plural
+// or Invalid and a non-nil error if src is not a valid Plural.
+func NewPlural(src string) (Plural, error) {
+	switch src {
+	case "zero":
+		return Zero, nil
+	case "one":
+		return One, nil
+	case "two":
+		return Two, nil
+	case "few":
+		return Few, nil
+	case "many":
+		return Many, nil
+	case "other":
+		return Other, nil
+	}
+	return Invalid, fmt.Errorf("invalid plural category %s", src)
+}