I fixed it, thanks.
For anyone having similar problems and really want to use my solution:
./launcher ssh nt
cd /var/www/discourse
rake db:migrate
# check website and panic because it's still 500ing
^D
mv shared/standalone shared/standalone_old
mkdir shared/standalone
./launcher rebuild standalone
# wait...
# check and see: it's working again, but with an empty db, as expected
./launcher stop standalone
mv shared/standalone_old shared/standalone
./launcher start standalone
# heyaa, it's back!