archwiki: disable Lockdown extension

This restores public access to recent changes, page history etc. which was disabled in September 2019 due to misbehaving crawlers - see commits https://github.com/archlinux/archwiki/commit/f2c518d3df94e9ad6898fdd03cac982902f8ac36 and https://github.com/archlinux/archwiki/commit/8595e8d66141e608db794f1deef3e52cb36e5e78.

Let's hope that our robots.txt has been improved since then and the server load will not increase much. If it proves to be still a problem, this commit can be reverted and we will figure out better solution.

Original pull request on GitHub: https://github.com/archlinux/archwiki/pull/29