Many changes

A large proportion of these changes relate to changing the way
packages relate to derivations. Previously, a package at a given
revision had a single derivation. This was OK, but didn't account for
multiple architectures.

Therefore, these changes mean that a package has multiple derivations,
depending on the system of the derivation, and the target system.

There are multiple changes, small and large to the web interface as
well. More pages link to each other, and the visual display has been
improved somewhat.

guix-data-service/model/package-metadata.scm

@@ -9,6 +9,7 @@
   #:use-module (guix inferior)
   #:use-module (guix-data-service model utils)
   #:export (select-package-metadata
+            select-package-metadata-by-revision-name-and-version
             insert-package-metadata
             inferior-packages->package-metadata-ids))
 
@@ -22,6 +23,29 @@
                               ",")
                  ");"))
 
+(define (select-package-metadata-by-revision-name-and-version
+         conn revision-commit-hash name version)
+  (define query "
+SELECT package_metadata.synopsis, package_metadata.description,
+  package_metadata.home_page
+FROM package_metadata
+INNER JOIN packages
+  ON package_metadata.id = packages.package_metadata_id
+WHERE packages.id IN (
+  SELECT package_derivations.package_id
+  FROM package_derivations
+  INNER JOIN guix_revision_package_derivations
+    ON package_derivations.id =
+    guix_revision_package_derivations.package_derivation_id
+  INNER JOIN guix_revisions
+    ON guix_revision_package_derivations.revision_id = guix_revisions.id
+  WHERE guix_revisions.commit = $1
+)
+  AND packages.name = $2
+  AND packages.version = $3")
+
+  (exec-query conn query (list revision-commit-hash name version)))
+
 (define (insert-package-metadata metadata-rows)
   (string-append "INSERT INTO package_metadata "
                  "(sha1_hash, synopsis, description, home_page) "