Added site settings for preventing unwhitelisted user messages and downvotes

In src/Controller, I changed the VoteController and MessageController to utilise the site settings to decide if the action is prevented if a user is not whitelisted and is not an admin. I changed the site settings template at templates/site/settings.html.twig which adds check boxes for admins to enable this feature. Also, the new migration alters the sites table in the database, it adds two columns for the new site settings.