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/Select.less

@@ -0,0 +1,18 @@
+.Select {
+  display: inline-block;
+  vertical-align: middle;
+
+  select {
+    display: inline-block;
+    width: auto;
+    -webkit-appearance: none;
+    -moz-appearance: none;
+    padding-right: 30px;
+    cursor: pointer;
+  }
+}
+.Select-caret {
+  margin-left: -30px;
+  pointer-events: none;
+  color: @control-color;
+}