FIX: return 400 for missing required params (#6546)

If a required param is missing return a 400 and show a message
displaying which param was missing. Added this to the application
controller so that we don't have to add this logic to every controller
action.

app/controllers/application_controller.rb

@@ -219,6 +219,10 @@ class ApplicationController < ActionController::Base
     render_json_error I18n.t('read_only_mode_enabled'), type: :read_only, status: 503
   end
 
+  rescue_from ActionController::ParameterMissing do |e|
+    render_json_error e.message, status: 400
+  end
+
   def redirect_with_client_support(url, options)
     if request.xhr?
       response.headers['Discourse-Xhr-Redirect'] = 'true'