Make `config.serve_static_assets` settable via an ENV var

`config.serve_static_assets` can now be set from an environment variable in
production mode. The feature remains off by default, but can be enabled by
setting `RAILS_SERVE_STATIC_FILES` to a non-empty string at boot time.

[Richard Schneeman, Godfrey Chan]
Deprecate `config.serve_static_assets`

Deprecate `config.serve_static_assets` in favor of `config.serve_public_assets`
to clarify that the option is unrelated to the asset pipeline.
Merge pull request #18100 from chancancode/serve_static_files

Allow static asset serving from env variable (enhanced!)