Expandable profile pictures

(This suggestion was raised in the packet post mortem on 9/25.)

The profile pictures shown in packet are a bit too small to be useful in helping recognize people. It'd be nice if there was an option to enlarge them. Maybe you'd tap on the image to see a larger view in a dialog? This should support both mobile and desktop users and it needs to apply to both upperclassmen and freshman use cases.

尚未确定任何文件、测试或入口点。定位高年级学生和新生视图中渲染头像的位置，然后追踪现有的响应式桌面端和移动端呈现方式；完成的标准是两种使用场景都能在不干扰周围页面的情况下，在对话框中打开更大的图片。

由索引模型根据 Issue 内容生成。