General token authentication (pages rss)

What do you think about this? It would save some extra configuration work and would also work for multi-foodcoop installations safely. A one-way hashing function is used ([HMAC SHA1](https://github.com/rails/rails/blob/6e2b734f56e4c789763b9a8e3922272a004a325f/activesupport/lib/active_support/message_verifier.rb)) which makes sure the secret is not exposed (same thing as cookie check).