Refactor task-card.hbs to remove potential bug

From https://github.com/code-corps/code-corps-ember/pull/1488#issuecomment-350194545

>It looks like the `selected/unselected` icons are "trapped" behind `canAssign`, or at the very least, written in a way that it appears they are.

>I think, with these tests (implemented in #1488) in place now, the `task-card.hbs` warrants a bit of a refactor to make it clearer.

从 task-card.hbs 开始，查看 pull request #1488 中实现的测试，然后阅读所引用的、关于疑似条件结构的 issue 评论。完成的标准是模板能更清楚地表示已选中/未选中的图标，并且现有测试继续通过。

由索引模型根据 Issue 内容生成。