css: refactor element positioning

No more javascript to change css properties
Remove a number of useless tables
Try to stop positioning elements with absolute, but use flex-boxes instead

Adds comment to pad template, and move popups and chatbox inside editorcontainerbox (so absolute positioning is straightforward)

Make the design more consistent: always use base color, font-family and font-size. USe relative font size if necessary (.9rem instead of 11px for example)

Remove two columns in the popups, just use one column

Remove css meant to support old browser (like -webkit-box-shadow, -moz-box-shadow). Those css rules are quite common now, and If we want to support very old browser, we should use clean-css or other tools to add them automatically

src/static/css/pad/loadingbox.css

@@ -55,4 +55,8 @@
     -webkit-transform: rotate(359deg);
     transform: rotate(359deg);
   }
+}
+
+#passwordRequired, #permissionDenied, #wrongPassword, #noCookie {
+  display:none;
 }