Create extension pg_trgm in the NixOS module

The creation of the `pg_trgm` extension needs superuser power. So,
this patch makes the extension creation in the Hydra NixOS module when
a local database is used.

If it is not possible to create this extension (remote database for
instance with nosuperuser), the creation of the `pg_trgm` index is
skipped (this index speedup queries on builds.drvpath) and warnings
are emitted:

    initialising the Hydra database schema...
    WARNING:  Can not create extension pg_trgm: permission denied to create extension "pg_trgm"
    WARNING:  HINT: Temporary provide superuser role to your Hydra Postgresql user and run the script src/sql/upgrade-57.sql
    WARNING:  The pg_trgm index on builds.drvpath has been skipped (slower complex queries on builds.drvpath)

This allows to keep smooth migrations: the migration process doesn't
require a manual step (but this manual step is recommended on big
remote databases).

hydra-module.nix

@@ -273,6 +273,7 @@ in
               runuser -u ${config.services.postgresql.superUser} -- ${config.services.postgresql.package}/bin/createdb -O hydra hydra
               touch ${baseDir}/.db-created
             fi
+            echo "create extension if not exists pg_trgm" | runuser -u ${config.services.postgresql.superUser} -- ${config.services.postgresql.package}/bin/psql hydra
           ''}
 
           if [ ! -e ${cfg.gcRootsDir} ]; then
@@ -415,6 +416,8 @@ in
         hydra-users hydra-queue-runner hydra
         hydra-users hydra-www hydra
         hydra-users root hydra
+        # The postgres user is used to create the pg_trgm extension for the hydra database
+        hydra-users postgres postgres
       '';
 
     services.postgresql.authentication = optionalString haveLocalDB