Fix error 500s creating new projects due to empty weights

GitLab 13.1.0 added the application_settings.repository_storages_weighted column, but it was possible that the value was not seeded properly due to a stale schema cache due to the use of ActiveRecord inside the migrations. Admins that upgraded might see 500 errors when creating projects.

To ensure that the data is seeded properly, we surgically add a migration that seeds the data only if it is not present. The schema version was intentionally used to avoid backporting conflicts and to ensure any background migrations will have the correct data when they run. The changes were largely taken from !35814 (merged).

Closes #225203 (closed)