Update rocket_i18n and add gettext_macros (#431)

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

templates/errors/unprocessable_entity.rs.html

@@ -3,7 +3,7 @@
 
 @(ctx: BaseContext)
 
-@:base(ctx, "Unprocessable entity", {
+@:base(ctx, "Unprocessable entity".to_string(), {
   <h1>@i18n!(ctx.1, "The content you sent can't be processed.")</h1>
   <p>@i18n!(ctx.1, "Maybe it was too long.")</p>
 })