Modify the submit handler to allow blocks-inside-blocks

[erral]

Currently the form submit endpoint checks if a block is really in the context, but to check that it only takes into account the blocks attribute of the context, meaning that if you have a block inside another block (for instance in a column, or a grid), the check will fail and the form can't be submitted.

@gorkafernandez

[mbarde]

Should be fixed via: https://github.com/collective/collective.volto.formsupport/pull/29