* configure.ac: Add Guile module check.

This is superseeded by the notification system.

* doc/cuirass.texi (Specification): Document the channels build type.

* src/cuirass/specification.scm (%build-types): Add 'channels support.

* src/cuirass/http.scm (url-handler): Remove admin section.
* src/cuirass/templates.scm (specifications-table): Remove optional admin
argument.

* bin/evaluate.in: Fix it.

* src/cuirass/specification.scm (sexp->channel): Ignore channel introduction.

The Guix version of sexp->channel doesn't support the branch field.

* src/cuirass/specification.scm (sexp->channel): New procedure.
* src/cuirass/database.scm (db-get-specifications): Use it.

* bin/evaluate.in: Fix it.

* bin/evaluate.in (checkouts->channel-instances): Remove it.
(instances->cached-profile*): Rename it into ...
(channels->cached-profile): ... this procedure.
(main): Adapt it.

* src/cuirass/specification.scm (channel->sexp): Add channel introduction
support.

* src/cuirass/specification.scm (<build-output>): Set a default path to "".

* src/sql/upgrade-2.sql: Remove it.
* src/sql/upgrade-3.sql: Remove it.
* src/sql/upgrade-4.sql: Remove it.
* src/sql/upgrade-5.sql: Remove it.
* Makefile.am (dist_sql_DATA): Adapt it.