cache local instance (#572)

* cache local instance

fix #564

* don't use local instance cache for plm

* use instance cache for plm, but initialize it

* cargo fmt

templates/medias/details.rs.html

@@ -14,7 +14,7 @@
 
     <section>
         <figure class="media">
-            @Html(media.html(ctx.0).unwrap_or_else(|_| SafeString::new("")))
+            @Html(media.html().unwrap_or_else(|_| SafeString::new("")))
             <figcaption>@media.alt_text</figcaption>
         </figure>
         <div>
@@ -22,7 +22,7 @@
                 @i18n!(ctx.1, "Markdown syntax")
                 <small>@i18n!(ctx.1, "Copy it into your articles, to insert this media:")</small>
             </p>
-            <code>@media.markdown(ctx.0).unwrap_or_else(|_| SafeString::new(""))</code>
+            <code>@media.markdown().unwrap_or_else(|_| SafeString::new(""))</code>
         </div>
         <div>
 	    @if media.category() == MediaCategory::Image {