refactor templates layout

@see https://gohugo.io/templates/new-templatesystem-overview/

layouts/_partials/media/video.html

@@ -0,0 +1,26 @@
+<!--
+    Render a video in an interactive player.
+
+    @params context Video media file.
+    @params page_file_path  Filepath of the calling file for error messages.
+-->
+
+{{- $src := .src -}}
+
+<video
+    class="u-video figure__video"
+    controls
+    itemprop="video"
+    preload="none"
+    src="{{ $src.RelPermalink }}"
+    poster="
+    {{- if $src.Params.poster -}}
+        {{- $posterSrc := $.Page.Resources.GetMatch ( .Get $src.Params.poster ) -}}
+        {{- partialCached "images/get-image.html" ( dict "img" $posterSrc ) $posterSrc.Title -}}
+    {{- else -}}
+        {{- erroridf "a11y-video-poster" "No poster defined for resource %q (%q)" $src.RelPermalink .page_file_path -}}
+    {{- end -}}
+    "
+>
+    Your browser doesn't support embedded video, <a href="{{ $src.RelPermalink }}">view the video here</a>.
+</video>