Editor improvements (#486)

- Make it possible to insert new paragraphs in the article body
- Make it impossible to copy formatted HTML (to make media insertion from markdown code work correctly)

TODO:

- [x] make it possible to escape draft mode
- [x] display errors from the server
- [x] button to go back to the "normal" editor
- [x] Avoid publishing placeholders

templates/posts/new.rs.html

@@ -17,6 +17,7 @@
       <header>
         <button id="publish" class="button">@i18n!(ctx.1, "Publish")</button>
         <p id="char-count">@content_len</p>
+        <a href="#" id="close-editor">@i18n!(ctx.1, "Classic editor (any changes will be lost)")</a>
       </header>
     </div>
     @if let Some(article) = article {