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/Form.less

@@ -0,0 +1,15 @@
+.Form--centered {
+  text-align: center;
+
+  .FormControl, .Button {
+    margin: 0 auto;
+    text-align: center;
+    height: 50px;
+    padding: 15px 20px;
+    font-size: 15px;
+  }
+}
+
+.Form-group {
+  margin-bottom: 12px;
+}