Update rocket_i18n and add gettext_macros (#431)

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

templates/posts/new.rs.html

@@ -8,16 +8,10 @@
 @use routes::posts::NewPostForm;
 @use routes::*;
 
-@(ctx: BaseContext, blog: Blog, editing: bool, form: &NewPostForm, is_draft: bool, article: Option<Post>, errors: ValidationErrors, medias: Vec<Media>, content_len: u64)
+@(ctx: BaseContext, title: String, blog: Blog, editing: bool, form: &NewPostForm, is_draft: bool, article: Option<Post>, errors: ValidationErrors, medias: Vec<Media>, content_len: u64)
 
-@:base(ctx, &i18n!(ctx.1, if editing { "Edit {0}" } else { "New post" }; &form.title), {}, {}, {
-    <h1>
-        @if editing {
-            @i18n!(ctx.1, "Edit {0}"; &form.title)
-        } else {
-            @i18n!(ctx.1, "Create a new post")
-        }
-    </h1>
+@:base(ctx, title.clone(), {}, {}, {
+    <h1>@title</h1>
     @if let Some(article) = article {
 	    <form id="post-form" class="new-post" method="post" action="@uri!(posts::update: blog = blog.actor_id, slug = &article.slug)" content-size="@content_len">
     } else {
@@ -27,7 +21,7 @@
         @input!(ctx.1, subtitle (optional text), "Subtitle", form, errors.clone(), "")
 
         @if let Some(ValidationErrorsKind::Field(errs)) = errors.clone().errors().get("content") {
-            @format!(r#"<p class="error">{}</p>"#, i18n!(ctx.1, &*errs[0].message.clone().unwrap_or(Cow::from("Unknown error"))))
+            @format!(r#"<p class="error">{}</p>"#, errs[0].message.clone().unwrap_or(Cow::from("Unknown error")))
         }
 
         <label for="plume-editor">@i18n!(ctx.1, "Content")<small>@i18n!(ctx.1, "Markdown syntax is supported")</small></label>