Description of a category item does not change color on selection

# Problem

From https://github.com/code-corps/code-corps-ember/pull/1079#discussion_r102404681

>The `description` text is always using the `$text--light` (`#666`) color and does not switch to `$default-color` (`#08A8FC` when selected.

The PR introduced a minor bug. The `iconClass` is bound to the icon element alone and nothing is bound to the `p` element which holds the description. That means that the `p.selected` style can no longer apply.

The component itself should be bound to the `iconClass` class property and styles should be rewritten so they apply to both the icon and the `p` element.

從 category-item 元件以及受 PR #1079 影響的樣式開始，接著檢查 iconClass 屬性如何繫結至圖示，以及 description p 元素如何套用樣式。當選取項目時，圖示和描述文字都會變成所選的顏色，即表示工作完成。

由索引模型根據 Issue 內容生成。