posts: add /posts/{id}.html?variant=tooltip template.

app/controllers/application_controller.rb

@@ -9,6 +9,7 @@ class ApplicationController < ActionController::Base
   before_action :set_started_at_session
   before_action :api_check
   before_action :set_safe_mode
+  before_action :set_variant
   # before_action :secure_cookies_check
   layout "default"
   helper_method :show_moderation_notice?
@@ -184,6 +185,10 @@ class ApplicationController < ActionController::Base
     end
   end
 
+  def set_variant
+    request.variant = params[:variant].try(:to_sym)
+  end
+
   User::Roles.each do |role|
     define_method("#{role}_only") do
       if !CurrentUser.user.is_banned_or_ip_banned? && CurrentUser.user.__send__("is_#{role}?")