Avoid ActionView::MissingTemplate occurred in sessions#new

Some browser request /:foodcoop/login with the HTTP-Accept-Header set
to "image/webp,image/*;q=0.8", which leads to an internal server error
due to a not existing template. Call respond_to to allow only html and
respond with the correct "406 Not Acceptable" HTTP status code.

app/controllers/sessions_controller.rb

@@ -4,6 +4,9 @@ class SessionsController < ApplicationController
   layout 'login'
 
   def new
+    respond_to do |format|
+      format.html
+    end
   end
 
   def create