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.

less/lib/AlertManager.less

@@ -0,0 +1,12 @@
+.AlertManager {
+  position: fixed;
+  bottom: 20px;
+  left: 20px;
+  z-index: @zindex-alerts;
+
+  .Alert {
+    display: inline-block;
+    margin-top: 20px;
+    .box-shadow(0 2px 6px @shadow-color);
+  }
+}