Remove most references to hydra.gnu.org.

Remove most references to hydra.gnu.org.

* Makefile.am (assert-binaries-available): Don't mention hydra.gnu.org
in comment.
* build-aux/check-available-binaries.scm: Likewise.
* build-aux/check-final-inputs-self-contained.scm: Likewise.
* doc/guix.texi (SUBSTITUTE-SERVER): New variable.
Use it throughout instead of "mirror.hydra.gnu.org".
* doc/contributing.texi (Submitting Patches): Likewise.
* gnu/services/base.scm (hydra-key-authorization)
(guix-activation): Remove mentions of "hydra.gnu.org" in comments and
messages.
* gnu/system/install.scm (%installation-services): Likewise.
* guix/scripts/size.scm (guix-size): Likewise.