Turn on belongs_to_required_by_default (rails 5.0 default).

[thomasdziedzic-pd]

As mentioned in https://github.com/lobsters/lobsters/pull/498/files#diff-b1fe55db50c712fef0673345e5b9c0d9R30

[pushcx]

Reviewing issues... how did this not break/start throwing warnings when I merged the config.load_defaults 5.2?

[thomasdziedzic]

I turn it off right after loading the 5.2 defaults: https://github.com/lobsters/lobsters/blob/master/config/application.rb#L29