Get rid of deprecation related to Zeitwerk autoloader.
Original PR was reverted because of multisite bug #12381 - thank you @davidtaylorhq for fixing it.
I added the last commit to fix that multisite problem.
Rails calls I18n.translate during initialization and by default translation overrides are used. Database migrations would fail if the system tried to migrate from an old version that didn't have the `translation_overrides` table with all its columns yet.
This makes restoring really old backups work again. Running `DISABLE_TRANSLATION_OVERRIDES=1 rake db:migrate` will allow you to upgrade such an old database as well.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
Without forcing a reload on start internal state in the accelerator can be
off. In Rails 5 not translation is being called so this is not an issue but
in 6 it is called earlier on.
