Move e-mail digest task to sidekiq, reduce workload, improve hint (#6252)

config/locales/simple_form.en.yml

@@ -4,7 +4,7 @@ en:
     hints:
       defaults:
         avatar: PNG, GIF or JPG. At most 2MB. Will be downscaled to 120x120px
-        digest: Sent after a long period of inactivity with a summary of mentions you've received in your absence
+        digest: Only sent after a long period of inactivity and only if you have received any personal messages in your absence
         display_name:
           one: <span class="name-counter">1</span> character left
           other: <span class="name-counter">%{count}</span> characters left

config/sidekiq.yml

@@ -27,3 +27,6 @@
   ip_cleanup_scheduler:
     cron: '<%= Random.rand(0..59) %> <%= Random.rand(3..5) %> * * *'
     class: Scheduler::IpCleanupScheduler
+  email_scheduler:
+    cron: '0 10 * * 2'
+    class: Scheduler::EmailScheduler