Modernize form JavaScript

[dannyvankooten]

Goals:

• No JavaScript by default, unless AJAX is enabled. See Port AJAX enabled forms from MC4WP Premium to free #835.
• Native browser events using addEventListener().
• We should have an upgrade path for users using the current JS API. Grace period, deprecation warnings, etc.
• Conditional elements JS printed inline in case form is using it. Otherwise, not loaded at all.