PLT-1800 Load server side locale from the config.json (#3135)

* PLT-1800 Load server side locale from the config.json

* Add support for locales with country specifics

* Fix localization on served locale file as plain/text

* Remove github.com/cloudfoundry/jibber_jabber as vendor dependency

api/api.go

@@ -10,7 +10,6 @@ import (
 	"github.com/mattermost/platform/model"
 	"github.com/mattermost/platform/utils"
 
-	_ "github.com/cloudfoundry/jibber_jabber"
 	_ "github.com/nicksnyder/go-i18n/i18n"
 )