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

config/config.json

@@ -110,7 +110,10 @@
         "PasswordResetSalt": "vZ4DcKyVVRlKHHJpexcuXzojkE5PZ5eL",
         "SendPushNotifications": false,
         "PushNotificationServer": "",
-        "PushNotificationContents": "generic"
+        "PushNotificationContents": "generic",
+        "EnableEmailBatching": false,
+        "EmailBatchingBufferSize": 256,
+        "EmailBatchingInterval": 30
     },
     "RateLimitSettings": {
         "EnableRateLimiter": true,