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/LoadingIndicator.less

@@ -0,0 +1,14 @@
+// ------------------------------------
+// Loading Indicators
+
+.LoadingIndicator {
+  position: relative;
+  color: @muted-color;
+}
+.LoadingIndicator--inline {
+  display: inline-block;
+  width: 25px;
+}
+.LoadingIndicator--block {
+  height: 100px;
+}