New editor (#458)

With this PR, when JS is activated and WASM supported, the article editor will be dynamically replaced with `contenteditable`s elements. This makes the editing interface simpler and less like a regular form. It will also allow us to easily add visual formatting with native browser APIs (and to insert images or videos directly). Here is a little demo:

![peek 05-03-2019 16-12](https://user-images.githubusercontent.com/16254623/53815536-1dc05680-3f62-11e9-94d3-b363ed84eb97.gif)

There is still a lot to do, but it is a good first step.

Fixes #255

po/plume-front/fr.po

@@ -0,0 +1,45 @@
+msgid ""
+msgstr ""
+"Project-Id-Version: plume-front\n"
+"Report-Msgid-Bugs-To: \n"
+"POT-Creation-Date: 2018-06-15 16:33-0700\n"
+"PO-Revision-Date: 2018-06-15 16:33-0700\n"
+"Last-Translator: Automatically generated\n"
+"Language-Team: none\n"
+"Language: fr\n"
+"MIME-Version: 1.0\n"
+"Content-Type: text/plain; charset=UTF-8\n"
+"Content-Transfer-Encoding: 8bit\n"
+"Plural-Forms: nplurals=2; plural=(n > 1);\n"
+
+# plume-front/src/editor.rs:57
+msgid "Title"
+msgstr "Titre"
+
+# plume-front/src/editor.rs:61
+msgid "Subtitle or summary"
+msgstr "Sous-titre ou résumé"
+
+# plume-front/src/editor.rs:68
+msgid "Write your article here. Markdown is supported."
+msgstr "Écrivez votre article ici. Vous pouvez utiliser du Markdown."
+
+# plume-front/src/editor.rs:46
+msgid "Around {} characters left"
+msgstr ""
+
+# plume-front/src/editor.rs:96
+msgid "Tags"
+msgstr ""
+
+# plume-front/src/editor.rs:97
+msgid "License"
+msgstr ""
+
+# plume-front/src/editor.rs:100
+msgid "Cover"
+msgstr ""
+
+# plume-front/src/editor.rs:111
+msgid "Publish"
+msgstr "Publier"