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/Slidable.less

@@ -0,0 +1,49 @@
+.Slidable-underneath {
+  display: none;
+}
+.Slidable {
+  position: relative;
+
+  .DiscussionListItem-controls {
+    display: block;
+    position: static;
+  }
+  .Slidable-underneath {
+    display: none;
+    background: @secondary-color !important;
+    position: absolute;
+    right: 0;
+    top: 0;
+    bottom: 0;
+    left: 0;
+    width: auto;
+    height: auto;
+    z-index: 0;
+    color: #fff !important;
+    border: 0;
+    border-radius: 0;
+    .box-shadow(none);
+    padding: 20px 0;
+    text-align: right;
+
+    .icon {
+      width: 50px;
+      text-align: center;
+      font-size: 20px;
+    }
+  }
+  .Slideable-underneath--left {
+    text-align: left;
+  }
+  .Slidable-content {
+    .transition(~"box-shadow 0.2s, border-radius 0.2s");
+
+    .sliding& {
+      position: relative;
+      background: #fff;
+      z-index: 2;
+      border-radius: 2px;
+      .box-shadow(0 2px 6px @shadow-color);
+    }
+  }
+}