services: tailon: Move to (gnu services web).

services: tailon: Move to (gnu services web).

This allows (gnu services admin) to remain deeper in the module graph
and to be used by (gnu services web).

* gnu/services/admin.scm (<tailon-configuration-file>)
(tailon-configuration-files-string)
(tailon-configuration-file-compiler, <tailon-configuration>)
(tailon-shepherd-service, %tailon-accounts)
(tailon-service-type): Move to...
* gnu/services/web.scm: ... here.
* gnu/tests/admin.scm: Remove.  Move test to...
* gnu/tests/web.scm (%tailon-os)
(run-tailon-test, %test-tailon): ... here.