Fix SAML redirect to with certain URLs e.g. /collection/root (#15410)

* Fix SAML redirect to /collection/root/

* Test util HTTP client should automatically URL-encode query parameters.

* Simplify HTTP client

* Minor tweaks

* Remove *url-encode-query-parameters?*

* experimental CI fix: cache build script deps

* Fix typo

* Fix Circle config again

* Fetch BE deps if deps.edn files change

* Use CI executor for be-deps, so we have the Clojure CLI