Slightly improve the media experience (#452)

* Slightly improve the media experience

- Use a grid to display the list of media
- Add icons for non-image media preview
- Paginate the gallery
- Add links to the gallery in the editor and in the profile settings to make it more discoverable when you need it

Fixes #432

* Allow video and audio tags in SafeString

Otherwise we can't display their preview, nor show them in articles

Also show controls by default for these two elements

* Show fallback images for audio and unknown files, to make them more visible

* Add a new constructor to SafeString when the input is trusted and doesn't need to be escaped.

And use it to generate media previews.

* Make it possible to insert video/audio in articles

templates/posts/new.rs.html

@@ -27,6 +27,10 @@
         <label for="plume-editor">@i18n!(ctx.1, "Content")<small>@i18n!(ctx.1, "Markdown syntax is supported")</small></label>
         <textarea id="plume-editor" name="content" rows="20">@form.content</textarea>
 	    <small id="editor-left">@content_len</small>
+        <p>
+            @i18n!(ctx.1, "You can upload medias to your gallery, and copy their Markdown code in your articles to insert them.")
+            <a href="@uri!(medias::new)">@i18n!(ctx.1, "Upload media")</a>
+        </p>
 
         @input!(ctx.1, tags (optional text), "Tags, separated by commas", form, errors.clone(), "")