Allow for displaying has_many in panels.

Proposed solution for #2659.
Fix nested has_many styling inside panel HMs
Raise with helpful message if using unsupported behavior
¯\(ツ)/¯

Everything kinda works here, but it's just getting really messy.
Here I try to make use of the `@already_in_an_inputs_block` to render
an `li` instead of a `div` to contain everything. New nested `has_many`
fieldsets should get wrapped in `li`, which they do, but it would be
better if there were a couple levels of `@already_in`. It's :turtle:s
all the way down.
Try out @igorbernstein's implementation

Still doesn't work.
Fix has_many nested fieldset indexing

Basically all I do here is let form_builder do it's thing with
it's weird invalid html, but account for it in the has_many.js
by counting both children('fieldsets') and all the fieldsets that
would have been originally rendered by the form inside the `li` tag.