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.

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