Store and display the location of packages

Store the location a package can be found at, and display this on the package
page.

If available, link off to the git repository containing the package.

sqitch/deploy/add_git_repositories_cgit_url_base.sql

@@ -0,0 +1,7 @@
+-- Deploy guix-data-service:add_git_repositories_cgit_url_base to pg
+
+BEGIN;
+
+ALTER TABLE git_repositories ADD COLUMN cgit_url_base character varying;
+
+COMMIT;