Updated to Lemmy 0.18.3

Release notes: https://github.com/LemmyNet/lemmy/blob/main/RELEASES.md

TL;DR it's faster, the database is smaller (my DB backups went from 3.5GB to 570MB!), and it's more secure.

Messed up a simple config and it took about an hour to get the site back up instead of 10 minutes as I had thought. Sorry!