Improve e-mail digest (#9689)

- Reduce time-to-digest from 20 to 7 days
- Fetch mentions starting from +1 day since last login
- Fix case when last login is more recent than last e-mail
- Do not render all mentions, only 40, but show number in subject
- Do not send digest to moved accounts
- Do send digest to silenced accounts

app/workers/scheduler/email_scheduler.rb

@@ -5,6 +5,9 @@ class Scheduler::EmailScheduler
 
   sidekiq_options unique: :until_executed, retry: 0
 
+  FREQUENCY      = 7.days.freeze
+  SIGN_IN_OFFSET = 1.day.freeze
+
   def perform
     eligible_users.reorder(nil).find_each do |user|
       next unless user.allows_digest_emails?
@@ -15,11 +18,8 @@ class Scheduler::EmailScheduler
   private
 
   def eligible_users
-    User.confirmed
-        .joins(:account)
-        .where(accounts: { silenced: false, suspended: false })
-        .where(disabled: false)
-        .where('current_sign_in_at < ?', 20.days.ago)
-        .where('last_emailed_at IS NULL OR last_emailed_at < ?', 20.days.ago)
+    User.emailable
+        .where('current_sign_in_at < ?', (FREQUENCY + SIGN_IN_OFFSET).ago)
+        .where('last_emailed_at IS NULL OR last_emailed_at < ?', FREQUENCY.ago)
   end
 end