chore(makefile): allow migration to be specified in make migrate

The migration to migrate to can now be specified by setting the `migration`
Makefile variable on the command line, e.g. `make migrate migrations=0013`.

Makefile

@@ -53,7 +53,7 @@ make-migrations:
 	@docker compose run --rm web python manage.py makemigrations
 
 migrate:
-	@docker compose run --rm web python manage.py migrate
+	@docker compose run --rm web python manage.py migrate $(migration)
 
 collectstatic:
 	@docker compose run --rm web python manage.py collectstatic --noinput