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

static/css/_article.scss

@@ -273,6 +273,7 @@ main .article-meta {
     flex-direction: row-reverse;
     background: transparent;
     align-items: center;
+    justify-content: space-between;
     button {
       flex: 0 0 10em;
       font-size: 1.25em;