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/users/edit.rs.html

@@ -9,6 +9,10 @@
 @:base(ctx, i18n!(ctx.1, "Edit your account"), {}, {}, {
     @if let Some(u) = ctx.2.clone() {
         <h1>@i18n!(ctx.1, "Your Profile")</h1>
+        <p>
+            @i18n!(ctx.1, "To change your avatar, upload it in your gallery and select from there.")
+            <a href="@uri!(medias::new)">@i18n!(ctx.1, "Upload an avatar")</a>
+        </p>
         <form method="post" action="@uri!(user::update: _name = u.username.clone())">
             <!-- Rocket hack to use various HTTP methods -->
             <input type=hidden name="_method" value="put">