Rename flags_default_topics to reviewable_default_topics

This functionality regressed with the reviewable update. I took the
opporunity to rename it when fixing it for clarity.

db/migrate/20190508135348_rename_site_setting_default_topics.rb

@@ -0,0 +1,9 @@
+class RenameSiteSettingDefaultTopics < ActiveRecord::Migration[5.2]
+  def up
+    execute "UPDATE site_settings SET name = 'reviewable_default_topics' WHERE name = 'flags_default_topics'"
+  end
+
+  def down
+    execute "UPDATE site_settings SET name = 'flags_default_topics' WHERE name = 'reviewable_default_topics'"
+  end
+end