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/utils/constants.jsx

@@ -52,7 +52,9 @@ export const Preferences = {
     COLLAPSE_DISPLAY_DEFAULT: 'false',
     USE_MILITARY_TIME: 'use_military_time',
     CATEGORY_THEME: 'theme',
-    CATEGORY_FLAGGED_POST: 'flagged_post'
+    CATEGORY_FLAGGED_POST: 'flagged_post',
+    CATEGORY_NOTIFICATIONS: 'notifications',
+    EMAIL_INTERVAL: 'email_interval'
 };
 
 export const ActionTypes = keyMirror({