PLT-1800 Load server side locale from the config.json (#3092)

* 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

webapp/components/admin_console/admin_sidebar.jsx

@@ -292,6 +292,15 @@ export default class AdminSidebar extends React.Component {
                                         />
                                     }
                                 />
+                                <AdminSidebarSection
+                                    name='localization'
+                                    title={
+                                        <FormattedMessage
+                                            id='admin.sidebar.localization'
+                                            defaultMessage='Localization'
+                                        />
+                                    }
+                                />
                                 <AdminSidebarSection
                                     name='users_and_teams'
                                     title={