PERF: Dematerialize topic_reply_count (#9769)

* PERF: Dematerialize topic_reply_count

It's only ever used for trust level promotions that run daily, or compared to 0. We don't need to track it on every post creation.

* UX: Add symbol in TL3 report if topic reply count is capped

* DEV: Drop user_stats.topic_reply_count column

db/post_migrate/20200513185052_drop_topic_reply_count.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class DropTopicReplyCount < ActiveRecord::Migration[6.0]
+  DROPPED_COLUMNS ||= {
+    user_stats: %i{
+      topic_reply_count
+    }
+  }
+
+  def up
+    DROPPED_COLUMNS.each do |table, columns|
+      Migration::ColumnDropper.execute_drop(table, columns)
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end