retrieve fetch() from polyfill.io, replacing lib/fetch

[walaura]

ISSUE

lib/fetch doesn't polyfill formData at the moment which makes us dependant on reqwest for sending forms. This should be trivial to add considering reqwest does support formdata already

Steps to Reproduce

Sending a form using fetch() won't work

const fd = new FormData();
fd.append('test-key', 'test-value');

fetch('/test',{
    method: 'POST',
    body: fd
})

Actual Results (include screenshots)

Doesn't submit the form

Expected Results (include screenshots)

Does submit the form

URL

N/A

OS and Browser details

N/A