FEATURE: New way to dismiss new topics (#11927)

This is a try to simplify logic around dismiss new topics to have one solution to work in all places - dismiss all-new, dismiss new in a specific category or even in a specific tag.

db/migrate/20210131221311_create_dismissed_topic_users_table.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+class CreateDismissedTopicUsersTable < ActiveRecord::Migration[6.0]
+  def change
+    create_table :dismissed_topic_users do |t|
+      t.integer :user_id
+      t.integer :topic_id
+      t.datetime :created_at
+    end
+    add_index :dismissed_topic_users, %i(user_id topic_id), unique: true
+  end
+end

db/migrate/20210201034048_move_category_last_seen_at_to_new_table.rb

@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+class MoveCategoryLastSeenAtToNewTable < ActiveRecord::Migration[6.0]
+  def up
+    sql = <<~SQL
+      INSERT INTO dismissed_topic_users (user_id, topic_id, created_at)
+      SELECT users.id, topics.id, category_users.last_seen_at
+      FROM category_users
+      JOIN users ON users.id = category_users.user_id
+      JOIN categories ON categories.id = category_users.category_id
+      JOIN user_stats ON user_stats.user_id = users.id
+      JOIN user_options ON user_options.user_id = users.id
+      JOIN topics ON topics.category_id = category_users.category_id
+      WHERE category_users.last_seen_at IS NOT NULL
+      AND topics.created_at >= GREATEST(CASE
+                  WHEN COALESCE(user_options.new_topic_duration_minutes, :default_duration) = :always THEN users.created_at
+                  WHEN COALESCE(user_options.new_topic_duration_minutes, :default_duration) = :last_visit THEN COALESCE(users.previous_visit_at,users.created_at)
+                  ELSE (:now::timestamp - INTERVAL '1 MINUTE' * COALESCE(user_options.new_topic_duration_minutes, :default_duration))
+               END, user_stats.new_since, :min_date)
+      AND topics.created_at <= category_users.last_seen_at
+      ORDER BY topics.created_at DESC
+      LIMIT :max_new_topics
+    SQL
+    sql = DB.sql_fragment(sql,
+                          now: DateTime.now,
+                          last_visit: User::NewTopicDuration::LAST_VISIT,
+                          always: User::NewTopicDuration::ALWAYS,
+                          default_duration: SiteSetting.default_other_new_topic_duration_minutes,
+                          min_date: Time.at(SiteSetting.min_new_topics_time).to_datetime,
+                          max_new_topics: SiteSetting.max_new_topics)
+    DB.exec(sql)
+  end
+
+  def down
+    raise IrriversibleMigration
+  end
+end