Update rocket_i18n and add gettext_macros (#431)

Internationalization now uses proc-macros that generate the .pot file
automatically.

templates/instance/about.rs.html

@@ -5,7 +5,7 @@
 
 @(ctx: BaseContext, instance: Instance, admin: User, n_users: i64, n_articles: i64, n_instances: i64)
 
-@:base(ctx, i18n!(ctx.1, "About {0}"; instance.name.clone()).as_str(), {}, {}, {
+@:base(ctx, i18n!(ctx.1, "About {0}"; instance.name.clone()), {}, {}, {
     <h1>@i18n!(ctx.1, "About {0}"; instance.name)</h1>
     <section>
     @Html(instance.short_description_html)