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.

api4/api.go

@@ -12,7 +12,7 @@ import (
 	"github.com/mattermost/mattermost-server/services/configservice"
 	"github.com/mattermost/mattermost-server/web"
 
-	_ "github.com/nicksnyder/go-i18n/i18n"
+	_ "github.com/mattermost/go-i18n/i18n"
 )
 
 type Routes struct {