Fix failure in rake db:migrate on fresh install (#3443).

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.

config/initializers/system_user.rb

@@ -1,10 +0,0 @@
-require "securerandom"
-
-system = User.find_or_create_by!(name: Danbooru.config.system_user) do |user|
-  user.password = SecureRandom.base64(32)
-end
-
-unless system.is_moderator?
-  system.level = User::Levels::MODERATOR
-  system.save
-end