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/forum/UserPage.less

@@ -0,0 +1,11 @@
+
+.UserPage {
+  .UserCard-controls {
+    float: right;
+
+    .Dropdown-toggle .Button-icon {
+      display: none;
+    }
+  }
+}
+