FEATURE: Per-category default slow mode duration for topics.

When configured, all topics in the category inherits the slow mode
duration from the category's default.

Note that currently there is no way to remove the slow mode from the
topics once it has been set.

app/models/topic.rb

@@ -334,6 +334,7 @@ class Topic < ActiveRecord::Base
 
     if category_id_changed? || new_record?
       inherit_auto_close_from_category
+      inherit_slow_mode_from_category
     end
   end
 
@@ -1251,6 +1252,12 @@ class Topic < ActiveRecord::Base
     end
   end
 
+  def inherit_slow_mode_from_category
+    if self.category&.default_slow_mode_seconds
+      self.slow_mode_seconds = self.category&.default_slow_mode_seconds
+    end
+  end
+
   def inherit_auto_close_from_category(timer_type: :close)
     auto_close_hours = self.category&.auto_close_hours