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/views/notification_mailer/digest.html.haml

@@ -14,7 +14,7 @@
                           %tr
                             %td.column-cell.text-center.padded
                               %h1= t 'notification_mailer.digest.title'
-                              %p.lead= t('notification_mailer.digest.body', since: l(@since.to_date, format: :short), instance: site_hostname)
+                              %p.lead= t('notification_mailer.digest.body', since: l((@me.user_current_sign_in_at || @since).to_date, format: :short), instance: site_hostname)
                               %table.button{ align: 'center', cellspacing: 0, cellpadding: 0 }
                                 %tbody
                                   %tr

app/views/notification_mailer/digest.text.erb

@@ -1,6 +1,6 @@
 <%= raw t('application_mailer.salutation', name: display_name(@me)) %>
 
-<%= raw t('notification_mailer.digest.body', since: l(@since), instance: root_url) %>
+<%= raw t('notification_mailer.digest.body', since: l(@me.user_current_sign_in_at || @since), instance: root_url) %>
 <% @notifications.each do |notification| %>
 
 * <%= raw t('notification_mailer.digest.mention', name: notification.from_account.acct) %>