see #14773

Builds on #14775 to use XHRs for file upload progress in forms:

<script>
  import { uploadForm } from './upload.remote';
</script>

<form {...uploadForm} enctype="multipart/form-data">
  <input {...uploadForm.fields.file.as('file')} />
  <p>Progress: {(uploadForm.fields.file.progress() * 100).toFixed(2)}%</p>
  <button>Upload</button>
</form>

Messy diff because I can't stack PRs here.

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