PLT-3647 Email Batching (#3718)

* PLT-3647 Added config settings for email batching

* PLT-3647 Refactored generation of email notification

* PLT-3647 Added serverside code for email batching

* PLT-3647 Updated settings UI to enable email batching

* PLT-3647 Removed debug code

* PLT-3647 Fixed 0-padding of minutes in batched notification

* PLT-3647 Updated clientside UI for when email batching is disabled

* Go fmt

* PLT-3647 Changed email batching to be disabled by default

* Updated batched email message

* Added email batching toggle to system console

* Changed Email Notifications > Immediate setting to a 30 second batch interval

* Go fmt

* Fixed link to Mattermost icon in batched email notification

* Updated users to use 30 second email batching by default

* Fully disabled email batching when clustering is enabled

* Fixed email batching setting in the system console

* Fixed casing of 'Send Email notifications' -> 'Send email notifications'

* Updating UI Improvements for email batching (#3736)

* Updated text for notification settings and SiteURL.

* Prevented enabling email batching when SiteURL isn't set in the system console

* Re-added a couple debug messages

* Added warning text when clustering is enabled

webapp/components/admin_console/configuration_settings.jsx

@@ -69,7 +69,7 @@ export default class ConfigurationSettings extends AdminSettings {
                     helpText={
                         <FormattedMessage
                             id='admin.service.siteURLDescription'
-                            defaultMessage='The URL, including port number and protocol, from which users will access Mattermost. Leave blank to automatically configure based on incoming traffic.'
+                            defaultMessage='The URL, including post number and protocol, that users will use to access Mattermost. Leave blank to automatically configure based on incoming traffic.'
                         />
                     }
                     value={this.state.siteURL}