FEATURE: site setting for suppressing categories from digest email

app/models/topic.rb

@@ -339,6 +339,10 @@ class Topic < ActiveRecord::Base
 
     # Remove muted categories
     muted_category_ids = CategoryUser.where(user_id: user.id, notification_level: CategoryUser.notification_levels[:muted]).pluck(:category_id)
+    if SiteSetting.digest_suppress_categories.present?
+      muted_category_ids += SiteSetting.digest_suppress_categories.split("|").map(&:to_i)
+      muted_category_ids = muted_category_ids.uniq
+    end
     if muted_category_ids.present?
       topics = topics.where("topics.category_id NOT IN (?)", muted_category_ids)
     end

config/locales/server.en.yml

@@ -1195,6 +1195,7 @@ en:
     digest_topics: "The maximum number of topics to display in the email digest."
     digest_min_excerpt_length: "Minimum post excerpt in the email digest, in characters."
     delete_digest_email_after_days: "Suppress digest emails for users not seen on the site for more than (n) days."
+    digest_suppress_categories: "Suppress these categories from digest emails."
     disable_digest_emails: "Disable digest emails for all users."
 
     detect_custom_avatars: "Whether or not to check that users have uploaded custom profile pictures."

config/site_settings.yml

@@ -512,6 +512,9 @@ email:
   digest_min_excerpt_length: 100
   digest_topics: 20
   delete_digest_email_after_days: 365
+  digest_suppress_categories:
+    type: category_list
+    default: ''
   disable_digest_emails:
     default: false
     client: true

spec/models/topic_spec.rb

@@ -1296,6 +1296,16 @@ describe Topic do
       expect(Topic.for_digest(user, 1.year.ago, top_order: true)).to be_blank
     end
 
+    it "doesn't return topics from suppressed categories" do
+      user = Fabricate(:user)
+      category = Fabricate(:category)
+      Fabricate(:topic, category: category)
+
+      SiteSetting.digest_suppress_categories = "#{category.id}"
+
+      expect(Topic.for_digest(user, 1.year.ago, top_order: true)).to be_blank
+    end
+
     it "doesn't return topics from TL0 users" do
       new_user = Fabricate(:user, trust_level: 0)
       Fabricate(:topic, user_id: new_user.id)
@@ -1607,7 +1617,7 @@ describe Topic do
 
     topic.update_status('closed', true, user)
     topic.reload
-    
+
     expect(@topic_status_event_triggered).to eq(true)
   end
 end