Prototype for a WYSIWYG editor

We use pulldown-cmark in plume-front too now, but instead of using the provided
HTML renderer, we use a custom DOM renderer, which let us use contenteditable only where we want,
and which will allow us to add event listeners to provide a good contextual edition experience.

Also removed the character counter, as the API limits are almost unreachable.

static/css/_article.scss

@@ -411,7 +411,7 @@ main .article-meta {
     margin-top: 110px;
   }
 
-  #edition-area > *[contenteditable] {
+  #editor-title, #editor-subtitle, #editor-main > * {
     padding-left: 18px;
     border-left: 2px solid transparent;
     transition: border-left-color 0.1s ease-in;