Update loader component to use role=progressbar

[schne324]

In anticipation of ARIA 1.2, which disallows using aria-label on roleless <div /> elements, we should probably update the <Loader /> component to use a proper role.

Using the progress bar spec as a guideline, we should:

• add role=progressbar to the div generated by the loader component
• follow this specific guideline:

  The author SHOULD supply values for aria-valuenow, aria-valuemin, and aria-valuemax, unless the value is indeterminate, in which case the author SHOULD omit the aria-valuenow attribute. Authors SHOULD update these values when the visual progress indicator is updated. If the progressbar is describing the loading progress of a particular region of a page, the author SHOULD use aria-describedby to point to the status, and set the aria-busy attribute to true on the region until it is finished loading.

• test this out with VO, NVDA, and JAWS to ensure it works as expected

[schne324]

maybe we could pull in https://hang-tight.netlify.app 😎 ♻️