Fix existing repository_storages_weighted migrations

For instances that upgraded to GitLab 13.1.x, it's likely that the application_settings.repository_storages_weighted column did not properly update. This happened because the schema for ApplicationSetting may have been cached, so the newly-added column wasn't visible to the model.

To ensure the column always has the right data, we make the pre-deployment migration fill in the right data and flush the cache. This essentially does the same thing as the post-deployment migration, but it ensures that the application can properly run before post-deployment migrations have completed.

Closes #225588 (closed)