When opt-in is disabled it fails to validate checkout data, because it expects
mailpoet.optin to present. This PRs skips extending checkout data schema in case
the opt-in is disabled.
[MAILPOET-4004]
Build version of plugin doesn't contain assets/js folder.
We need to copy block.json file with the metadata to assets/dist directory and load it from there.
[MAILPOET-3920]