FEATURE: add option to always send excerpts in emails

Enable the new setting "post excerpts in emails" to send excerpts
instead of complete posts in notification emails. Control the length of
excerpts with the "post excerpt maxlength" setting.

app/views/email/notification.html.erb

@@ -5,7 +5,7 @@
   <%- if SiteSetting.private_email? %>
     <p><%= t('system_messages.contents_hidden') %></p>
   <% else %>
-    <%= render partial: 'email/post', locals: { post: post, use_excerpt: false } %>
+    <%= render partial: 'email/post', locals: { post: post, use_excerpt: SiteSetting.post_excerpts_in_emails } %>
 
     <% if in_reply_to_post.present? || context_posts.present? %>
       <div class='footer'>%{respond_instructions}</div>

config/locales/server.en.yml

@@ -1815,6 +1815,7 @@ en:
     always_show_trimmed_content: "Always show trimmed part of incoming emails. WARNING: might reveal email addresses."
     private_email: "Don't include content from posts or topics in email title or email body. NOTE: also disables digest emails."
     email_total_attachment_size_limit_kb: "Max total size of files attached to outgoing emails. Set to 0 to disable sending of attachments."
+    post_excerpts_in_emails: "In notification emails, always send excerpts instead of full posts"
 
     manual_polling_enabled: "Push emails using the API for email replies."
     pop3_polling_enabled: "Poll via POP3 for email replies."

config/site_settings.yml

@@ -1034,6 +1034,7 @@ email:
   email_total_attachment_size_limit_kb:
     default: 0
     max: 51200
+  post_excerpts_in_emails: false
 
 files:
   max_image_size_kb:

spec/mailers/user_notifications_spec.rb

@@ -336,6 +336,25 @@ describe UserNotifications do
       expect(mail.text_part.to_s).to_not include(response.raw)
       expect(mail.text_part.to_s).to_not include(topic.url)
     end
+
+    it "includes excerpt when post_excerpts_in_emails is enabled" do
+      paragraphs = [
+        "This is the first paragraph, but you should read more.",
+        "And here is its friend, the second paragraph."
+      ]
+      SiteSetting.post_excerpts_in_emails = true
+      SiteSetting.post_excerpt_maxlength = paragraphs.first.length
+      response.update_attributes!(raw: paragraphs.join("\n\n"))
+      mail = UserNotifications.user_replied(
+        user,
+        post: response,
+        notification_type: notification.notification_type,
+        notification_data_hash: notification.data_hash
+      )
+      mail_html = mail.html_part.body.to_s
+      expect(mail_html.scan(/#{paragraphs[0]}/).count).to eq(1)
+      expect(mail_html.scan(/#{paragraphs[1]}/).count).to eq(0)
+    end
   end
 
   describe '.user_posted' do