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.

test/test_helper.rb

@@ -64,3 +64,5 @@ end
 
 Delayed::Worker.delay_jobs = false
 TestAfterCommit.enabled = false
+
+Rails.application.load_seed