Reimplement response caching on login/logout routes

Problem/Motivation

Caching is nice. It keeps us from having to do hash calculations (signing a SAML message) on every login/logout request.

(Because SAML LoginRequests/LogoutRequests that we construct don't contain anything that has to be dynamic. We're fine caching them for, say, 10 minutes.)

Last time I tried, failed for any cached request to /saml/login / /saml/logout that contains a 'destination' parameter created endless loops. (Analysis in #3205960-4: Drupal route cache causing problems)

Proposed resolution

#3205960-7:

I should weigh our options: Either add our own response subscriber that is executed before RedirectResponseSubscriber and strips the 'destination' parameter... or implement our own kind of caching. (Of at least the SAML message including the computed hash - based on the input 'destination' parameter.) Probably the former.