Use form elements as key point of extensibility & adjust forms to be view concerns only

[timriley]

This is a pretty big restructure to achieve a few different things:

• Accept a Dry::Validation::Schema::Result as input, instead of having the form run the validation itself (this means that the form can remain a view concern only and the app can deal with the schema directly in other contexts)
• Center around "form elements" as the key point for form extensibility. Define the 6 main elements in a consistent way and allow them to be subclassed later (and included in a form definition via the DSL)
• Make the form definition DSL more natural by removing block arguments and having the form element name be the keyword for adding elements.
• Provide an optional standard library of elements that match our formalist-standard-react library.

[timriley]

This is looking good. Going to merge into master now so @makenosound can go ahead and update the UI libraries.