Theme update (#553)

* Start to update the theme

- Ligther colors
- No more border radius
- Buttons are now always colored
- Start to redesign the post page (according to the Figma mockups)

* Fix build script: it now recompiles everytime a scss file changed

* Make sure the article illustrations are not too big

* Make articles wider (70 characters)

* Better contrast between gray shades

* Various improvements

* Better mobile style

* New style for the footer

* Improve comment style

* Better responsiveness again

* Limit the size of the article cover

* Last details?

- Improve buttons on the media page
- Improve lists

* Pin the stdweb version that we use

It changed because I removed Cargo.lock to handle a merge conflict

I could have updated cargo web too, but it mean I should have re-built
the CI docker image and it was taking forever.

* Better contrast for links in the header of the article

* Add a basic privacy policy

* Remove "also"

* Fix a few issues

- Don't watch static/css in build.rs
- Another shade of white
- Remove useless margin rule for error messages

static/css/_header.scss

@@ -1,5 +1,5 @@
-header {
-  background: $lightgray;
+body > header {
+  background: $gray;
 
   #content {
     display: flex;
@@ -27,7 +27,7 @@ header {
      	height: 1.4em;
      	margin: 0;
      	padding: 0;
-     	color: $lightgray;
+     	color: $gray;
      	font-size: 1.33em;
    	}
   }