Add form-name hidden field for Netlify Forms

[veth-tech]

Description

1. Added name attribute to form element.
2. Added hidden input field with form-name

It appears that Gatsby strips the form name from the form submission. By adding the hidden field you can restore the name and don't run into submission issues with Netlify.

Documentation

Netlify Docs showing hidden field -> https://www.netlify.com/blog/2017/07/20/how-to-integrate-netlifys-form-handling-in-a-react-app/#step-2
Example code from -> https://codebushi.com/form-handling-gatsby-netlify/

Related Issues

Other have run into this issue at -> #12040

[marcysutton]

Thanks for the PR, makes sense to me! I already had that on my own site which uses Netlify Forms–so it does work.

[gatsbot]

Holy buckets, @veth-tech — we just merged your PR to Gatsby! 💪💜

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