download: Fix rendering of download page within slicer_dowload_server

[jcfr]

This commit is a follow-up of 769e146 (download: Introduce "download_table_td" template to simplify maintenance) fixing rendering of the download page by using keys matching values associated with SUPPORTED_OS_CHOICES and STABILITY_CHOICES defined in slicer_dowload_server.

Faulty commit was introduced by this pull-request:

• https://github.com/Slicer/slicer.org/pull/142

Regression details

• Server affected: download.slicer.org (and download-staging.slicer.org)
• Observed from 2023-03-21 01:15:15,788 to 2023-03-21 03:00:25,539 (ET, UTC -4)
• Hot fix similar to this pull-request manually applied to the server

Traceback (most recent call last):
  [...]
  File "/path/to/download-slicer-org/slicer_download/slicer_download_server/templates/download.html", line 483, in top-level template code
    <a href="{{R.win.stable.download_url}}" class="button-download expanded hollow ">
  File "/path/to/.../jinja2/environment.py", line 471, in getattr
    return getattr(obj, attribute)

Traceback copied from uwsgi.log and redacted by only keeping the last reference and removing irrelevant paths.

[slicerbot]

download.slicer.org preview

:warning: The download preview is a static website generated using mock data [^1], is temporary and may be updated at anytime [^2]	Screenshot from https://deploy-download-preview--slicer-org.netlify.app/download.html

[^1]: See front matter variable download_mock associated with https://raw.githubusercontent.com/Slicer/slicer.org/main/download.markdown [^2]: Due to limitation of Netlify preventing us from having multiple deploy previews associated with a single pull request and the impossibility of using repository secret in a workflow associated with a pull-request originating from forks, the deploy-download-preview site is only updated for pull request originating from this repository and will be overriden after another pull request is pushed or updated.