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/DiscussionList.less

@@ -0,0 +1,22 @@
+// ------------------------------------
+// Discussions List
+
+.DiscussionList-discussions {
+  margin: 0;
+  padding: 0;
+  list-style-type: none;
+  position: relative;
+}
+.DiscussionList-loadMore {
+  text-align: center;
+  margin-top: 10px;
+}
+.DiscussionList-loadMore .LoadingIndicator {
+  height: 46px;
+}
+
+@media @phone {
+  .DiscussionList {
+    margin: 0 -15px;
+  }
+}