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

build.rs

@@ -16,6 +16,7 @@ fn main() {
             .expect("Error during SCSS compilation")
     ).expect("Couldn't write CSS output");
 
+    println!("cargo:rerun-if-changed=target/deploy/plume-front.wasm");
     copy("target/deploy/plume-front.wasm", "static/plume-front.wasm")
         .and_then(|_| read_to_string("target/deploy/plume-front.js"))
         .and_then(|js| write("static/plume-front.js", js.replace("\"plume-front.wasm\"", "\"/static/plume-front.wasm\""))).ok();