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/main_test.go

@@ -13,7 +13,7 @@ var mainHelper *testlib.MainHelper
 
 func TestMain(m *testing.M) {
 	var options = testlib.HelperOptions{
-		EnableStore: true,
+		EnableStore:     true,
 		EnableResources: true,
 	}