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/de.po

@@ -542,6 +542,9 @@ msgstr ""
 msgid "Username or email"
 msgstr "Nutzername oder E-Mail"
 
+msgid "Publish"
+msgstr "Veröffentlichen"
+
 msgid "Subtitle"
 msgstr "Untertitel"
 
@@ -901,9 +904,6 @@ msgstr "Als Avatar verwenden"
 #~ msgid "Let's go!"
 #~ msgstr "Los geht's!"
 
-#~ msgid "Publish"
-#~ msgstr "Veröffentlichen"
-
 #~ msgid "One follower"
 #~ msgid_plural "{0} followers"
 #~ msgstr[0] "Ein Follower"