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.
Post-zeitwerk, rails has deprecated autoloading modules during
initialization and forces all autoloaded modules to be reloaded after
initialization.
Requiring the file explicitly prevents autoloading and therefore
prevents the state on SiteSetting being trashed which was causing the
problem here.
Sometimes we would like to create a base image without any DB access, this
assists in creating custom base images with custom plugins that already
includes `public/assets`
Following this change set you can run:
```
SPROCKETS_CONCURRENT=1 DONT_PRECOMPILE_CSS=1 SKIP_DB_AND_REDIS=1 RAILS_ENV=production bin/rake assets:precompile
```
Then it is straight forward to create a base image without needing a DB or
Redis.
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
