Add a staticCacheMode plugin setting for when your site uses a static cache

[engram-design]

While we have docs to guide users with static cached sites and Formie, it'd be great if you could set a plugin setting (environmentally, and for all forms) where Formie would "know" it needs to add handling for a static cache.

This would automatically refresh CSRF and captcha tokens without needing to include the JS. It means we no longer need to burden the developer with maintaining this JS.

There's also a few other things to enforce such as cache headers, handling page-reload vs Ajax, the console.log output

Related #1993, #1751, #1708