[ticket/11574] Fix more issues in the updater

* Stupid mistake in phpbb_create_update_container
* Do not bootstrap extensions in installer/updater
* Fix template lookup in installer/updater
* Do not attempt to delete posts from bots

The latter is a really fun problem. Since deleting posts now depends
on a new db column that does not exist yet, we cannot call delete_post
from a migration, ever. By using retain, we can hack around the issue
for now.

PHPBB3-11574