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.

sqitch/deploy/license_support.sql

@@ -0,0 +1,27 @@
+-- Deploy guix-data-service:license_support to pg
+
+BEGIN;
+
+CREATE TABLE licenses (
+    id integer NOT NULL GENERATED ALWAYS AS IDENTITY,
+    name character varying NOT NULL,
+    uri character varying,
+    comment character varying,
+    PRIMARY KEY(id),
+    UNIQUE (name, uri, comment)
+);
+
+CREATE TABLE license_sets (
+    id integer GENERATED ALWAYS AS IDENTITY,
+    license_ids integer[] NOT NULL,
+    PRIMARY KEY(license_ids),
+    UNIQUE (id)
+);
+
+ALTER TABLE package_metadata ADD COLUMN license_set_id integer REFERENCES license_sets(id);
+
+ALTER TABLE package_metadata DROP CONSTRAINT synopsis_description_home_page_location_id;
+
+ALTER TABLE package_metadata ADD CONSTRAINT package_metadata_unique_fields UNIQUE (synopsis, description, home_page, location_id, license_set_id);
+
+COMMIT;