rename atom shortcodes from .atom.xml to .atom.atom, and stop using symlinks

let's see if this fixes the issue of the wrong shortcode types (usually AMP) being chosen inconsistently

layouts/shortcodes/facebook.atom.atom

@@ -0,0 +1,11 @@
+{{- $url := printf "https://www.facebook.com/plugins/post/oembed.json/?omitscript=1&url=%s" (.Get "url" | urlize) -}}
+{{- $json := getJSON $url -}}
+
+<div class="embed facebook">
+{{ $json.html | safeHTML }}
+</div>
+
+{{- define "__shortcode_facebook_js" }}
+<div id="fb-root"></div>
+<script async defer src="https://connect.facebook.net/en_US/sdk.js#xfbml=1&version=v5.0"></script>
+{{- end }}