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/ja.po

@@ -533,6 +533,9 @@ msgstr "ログイン"
 msgid "Username or email"
 msgstr "ユーザー名またはメールアドレス"
 
+msgid "Publish"
+msgstr "公開"
+
 msgid "Subtitle"
 msgstr "サブタイトル"
 
@@ -814,9 +817,6 @@ msgstr "アバターとして使う"
 #~ msgid "Create a post"
 #~ msgstr "記事を作成"
 
-#~ msgid "Publish"
-#~ msgstr "公開"
-
 #~ msgid "One follower"
 #~ msgid_plural "{0} followers"
 #~ msgstr[0] "{0} フォロワー"