FIX: Respect muted tags for mailing list mode

If a user has a tag muted, don't send them emails about that tag.
We've done this forever for categories so it makes sense to do it
for tags too.

app/jobs/regular/notify_mailing_list_subscribers.rb

@@ -53,6 +53,14 @@ module Jobs
                      WHERE cu.category_id = ? AND cu.user_id = users.id AND cu.notification_level = ?
                   )', post.topic.category_id, CategoryUser.notification_levels[:muted])
 
+      if SiteSetting.tagging_enabled?
+        users = users.where('NOT EXISTS (
+           SELECT 1
+           FROM tag_users tu
+           WHERE tu.tag_id in (:tag_ids) AND tu.user_id = users.id AND tu.notification_level = :muted
+        )', tag_ids: post.topic.tag_ids, muted: TagUser.notification_levels[:muted])
+      end
+
       if SiteSetting.must_approve_users
         users = users.where(approved: true)
       end

spec/jobs/notify_mailing_list_subscribers_spec.rb

@@ -7,9 +7,14 @@ describe Jobs::NotifyMailingListSubscribers do
   fab!(:mailing_list_user) { Fabricate(:user) }
 
   before { mailing_list_user.user_option.update(mailing_list_mode: true, mailing_list_mode_frequency: 1) }
+  before do
+    SiteSetting.tagging_enabled = true
+  end
 
+  fab!(:tag) { Fabricate(:tag) }
+  fab!(:topic) { Fabricate(:topic, tags: [tag]) }
   fab!(:user) { Fabricate(:user) }
-  fab!(:post) { Fabricate(:post, user: user) }
+  fab!(:post) { Fabricate(:post, topic: topic, user: user) }
 
   shared_examples "no emails" do
     it "doesn't send any emails" do
@@ -127,6 +132,11 @@ describe Jobs::NotifyMailingListSubscribers do
         include_examples "no emails"
       end
 
+      context "from a muted tag" do
+        before { TagUser.create(user: mailing_list_user, tag: tag, notification_level: TagUser.notification_levels[:muted]) }
+        include_examples "no emails"
+      end
+
       context "max emails per day was reached" do
         before { SiteSetting.max_emails_per_day_per_user = 2 }