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/revert/add_git_repositories_cgit_url_base.sql

@@ -0,0 +1,7 @@
+-- Revert guix-data-service:add_git_repositories_cgit_url_base from pg
+
+BEGIN;
+
+ALTER TABLE git_repositories DROP COLUMN cgit_url_base;
+
+COMMIT;

sqitch/revert/add_location_information.sql

@@ -0,0 +1,7 @@
+-- Revert guix-data-service:add_location_information from pg
+
+BEGIN;
+
+-- XXX Add DDLs here.
+
+COMMIT;