Update rocket_i18n and add gettext_macros (#431)

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

templates/medias/details.rs.html

@@ -6,7 +6,7 @@
 
 @(ctx: BaseContext, media: Media)
 
-@:base(ctx, "Media details", {}, {}, {
+@: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>

templates/medias/index.rs.html

@@ -6,7 +6,7 @@
 
 @(ctx: BaseContext, medias: Vec<Media>)
 
-@:base(ctx, "Your media", {}, {}, {
+@:base(ctx, i18n!(ctx.1, "Your media"), {}, {}, {
     <h1>@i18n!(ctx.1, "Your media")</h1>
     <div>
         <a href="@uri!(medias::new)" class="inline-block button">@i18n!(ctx.1, "Upload")</a>

templates/medias/new.rs.html

@@ -4,7 +4,7 @@
 
 @(ctx: BaseContext)
 
-@:base(ctx, "Media upload", {}, {}, {
+@:base(ctx, i18n!(ctx.1, "Media upload"), {}, {}, {
     <h1>@i18n!(ctx.1, "Media upload")</h1>
     <form method="post" enctype="multipart/form-data" action="@uri!(medias::upload)">
         <label for="alt">