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/DiscussionHero.less

@@ -0,0 +1,19 @@
+.DiscussionHero {
+  .badges {
+    margin-right: 5px;
+    margin-bottom: -2px;
+  }
+}
+.DiscussionHero-items {
+  padding: 0;
+  margin: 0;
+  list-style: none;
+
+  & > li {
+    display: inline-block;
+  }
+}
+.DiscussionHero-title {
+  display: inline;
+  vertical-align: middle;
+}