Move to the mattermost/go-i18n fork (#10669)

This change is being made to address an issue where the go-i18n
translation library would result in partial-translations when a
given language dictionary was missing a given plural keyword. The
improvement made here leads the translation library to try an
'other' keyword lookup if the first plural keyword fails to have
a value.

This change was not accepted upstream due to concern regarding
changing the behavior, so we are using a fork at this time to
address the issue.

app/command_code.go

@@ -6,8 +6,8 @@ package app
 import (
 	"strings"
 
+	goi18n "github.com/mattermost/go-i18n/i18n"
 	"github.com/mattermost/mattermost-server/model"
-	goi18n "github.com/nicksnyder/go-i18n/i18n"
 )
 
 type CodeProvider struct {