Support non-gravatar pictures for freshmen

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

Allow freshmen to upload a profile picture directly to packet or alternatively, provide a link to the gravatar website so they can upload one there. (Watch out for caching issues with gravatar.) This should help upperclassmen match names to faces by reducing the number of freshmen without a photo.

首先跟踪 Packet 当前如何显示 freshman 的个人资料图片，并检查现有的 Gravatar 集成，包括其缓存行为。比较直接上传与将用户链接到 Gravatar 两种方式，然后确认 freshman 能够获取照片，并且 upperclassmen 能够可靠地看到更新后的图片。

由索引模型根据 Issue 内容生成。