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/AvatarEditor.less

@@ -0,0 +1,34 @@
+.AvatarEditor {
+  position: relative;
+
+  .Dropdown-toggle {
+    opacity: 0;
+    position: absolute;
+    left: 0;
+    top: 0;
+    right: 0;
+    bottom: 0;
+    border-radius: 100%;
+    background: rgba(0, 0, 0, 0.6);
+    text-align: center;
+    text-decoration: none;
+    border: 0;
+  }
+  &:hover .Dropdown-toggle, &.open .Dropdown-toggle, &.loading .Dropdown-toggle {
+    opacity: 1;
+  }
+  .LoadingIndicator {
+    color: #fff;
+    position: absolute;
+    left: 0;
+    top: 0;
+    right: 0;
+    bottom: 0;
+  }
+  @media @tablet-up {
+    .Dropdown-menu {
+      left: 35%;
+      top: 65%;
+    }
+  }
+}