PERF: speed up user deletion logic

Previously any user deletion would scan a very large number of tables

This avoids scans on 6 tables/indexes on delete

db/migrate/20190426074404_add_missing_user_destroyer_indexes.rb

@@ -0,0 +1,10 @@
+class AddMissingUserDestroyerIndexes < ActiveRecord::Migration[5.2]
+  def change
+    # these indexes are required to make deletions of users fast
+    add_index :user_actions, [:target_user_id], where: 'target_user_id IS NOT NULL'
+    add_index :post_actions, [:user_id]
+    add_index :user_uploads, [:user_id, :upload_id]
+    add_index :user_auth_token_logs, [:user_id]
+    add_index :topic_link, [:user_id]
+  end
+end