Add mailchimp newsletter form

[khalilcodes]

Tasks from #307

Summary

This PR adds lifeitself's mailchimp newsletter signup form in the layout for all pages (except podcast pages).

Changes

• Create custom components for newsletter - Newsletter.jsx, useMailchimp.js
  • Add jsonp package (to fetch/post to mailchimp)
• Add Newsletter form component in layout
• Add Mailchimp env variable in Netlify

Screenshot

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[nathenf]

@khalilcodes it looks great! And do the emails go automatically to mailchimp?

[nathenf]

@khalilcodes if so it looks good to go!

[khalilcodes]

@nathenf they do go but can you also test to confirm ? Also for the whatsapp link what number should be provided ?

[nathenf]

This comes up right now, but maybe because it isn't live? @khalilcodes

[khalilcodes]

@nathenf oh right didn't update the form in mailchimp to remove first name.

[khalilcodes]

@nathenf can you test again

[nathenf]

@khalilcodes yes this works now!

[nathenf]

@khalilcodes we are good to Merge!