Store license information for packages

And display this on the package page.

This uses a couple of new tables, and an additional field in the
package_metadata table.

Currently, the order of the licenses in the package definition isn't stored,
as I'm not sure the order in the list is significant.

tests/model-license.scm

@@ -0,0 +1,42 @@
+(define-module (tests model-license)
+  #:use-module (srfi srfi-64)
+  #:use-module (guix utils)
+  #:use-module (guix tests)
+  #:use-module (guix-data-service database)
+  #:use-module (tests mock-inferior)
+  #:use-module (guix-data-service model license))
+
+(test-begin "test-model-license")
+
+(mock
+ ((guix-data-service model license)
+  inferior-packages->license-data
+  (lambda (inf packages)
+    '((("License 1"
+        "https://gnu.org/licenses/test-1.html"
+        "https://example.com/why-license-1"))
+      (("License 1"
+        "https://gnu.org/licenses/test-1.html"
+        "https://example.com/why-license-1")
+       ("License 2"
+        "https://gnu.org/licenses/test-2.html"
+        "https://example.com/why-license-2")))))
+
+ (with-postgresql-connection
+  (lambda (conn)
+    (with-postgresql-transaction
+     conn
+     (lambda (conn)
+       (test-assert "works"
+         (inferior-packages->license-id-lists conn #f #f)))
+     #:always-rollback? #t)
+
+    (with-postgresql-transaction
+     conn
+     (lambda (conn)
+       (test-equal "works repeatedly"
+         (inferior-packages->license-id-lists conn #f #f)
+         (inferior-packages->license-id-lists conn #f #f)))
+     #:always-rollback? #t))))
+
+(test-end)