Add ignore_create_seed_file option

I don't want to update the seeds.rb in my application.

I add SeedMigration.ignore_create_seed_file.
If it's true, skip create_seed_file method.


I don't love this syntax, we try not to use `unless`, but more importantly having two return return statements one right after another seems unnecessary. Those two lines could be rewritten as:

```ruby
if !Rails.env.development? || !SeedMigration.update_seeds_file
    return
end
```