`User.find_or_create_by!` calls `User.name_matches` during username
validation, which triggers a deprecation warning for some reason:
DEPRECATION WARNING: Class level methods will no longer inherit scoping
from `create!` in Rails 6.1. To continue using the scoped relation,
pass it into the block directly. To instead access the full set of
models, as Rails 6.1 will, use `User.default_scoped`.
Create the DanbooruBot account in db/seeds.rb instead of in an
initializer. Move the old db/seeds.rb to db/populate.rb.
Fixes a bug introduced in e2eb45a. Creating DanbooruBot in an
initializer was wrong because in a fresh install the users table doesn't
exist yet. This caused `rake db:migrate` to fail.
