If migrations are pending, make CurrentSettings use existing values and populate missing columns with defaults

master was failing because `ApplicationSetting.create_from_defaults` attempted
to write to a column that did not exist in the database. This occurred in a
`rake db:migrate` task, which was unable to perform the migration that would
have added the missing column in the first place.

In 9.3 RC2, we also had a bug where password sign-ins were disabled because
there were many pending migrations. The problem occurred because
`fake_application_settings` was being returned with an OpenStruct that did not
include the predicate method `signup_enabled?`. As a result, the value would
erroneously return `nil` instead of `true`. This commit uses the values of the
defaults to mimic this behavior.

This commit also refactors some of the logic to be clearer.