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/medias/details.rs.html

@@ -9,7 +9,7 @@
 @:base(ctx, i18n!(ctx.1, "Media details"), {}, {}, {
     <h1>@i18n!(ctx.1, "Media details")</h1>
     <section>
-        <a href="@uri!(medias::list)">@i18n!(ctx.1, "Go back to the gallery")</a>
+        <a href="@uri!(medias::list: page = _)">@i18n!(ctx.1, "Go back to the gallery")</a>
     </section>
 
     <section>