Major CSS revamp

- Get rid of Bootstrap (except we still rely on some JS)
- Use BEM class names
- Rework variables/theme config
- Fix various bugs, including some on mobile

The CSS is still not ideal – it needs to be cleaned up some more. But
that can be a focus for after beta.

js/lib/utils/classList.js

@@ -10,10 +10,16 @@
  * @return {String}
  */
 export default function classList(classes) {
-  const classNames = [];
+  let classNames;
 
-  for (const i in classes) {
-    if (classes[i]) classNames.push(i);
+  if (classes instanceof Array) {
+    classNames = classes.filter(name => name);
+  } else {
+    classNames = [];
+
+    for (const i in classes) {
+      if (classes[i]) classNames.push(i);
+    }
   }
 
   return classNames.join(' ');