Prompt users for profile pictures

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

Add a reminder for users to upload a profile picture if they don't have one. This should apply to both upperclassmen and freshmen. This should be tied into the DB or `localStorage` so the user is only prompted once.

Start by tracing the existing profile-picture flow and the user prompt entry point; the issue requires coverage for both upperclassmen and freshmen. Add a one-time reminder for users without a profile picture, backed by the database or localStorage, and verify that users are not prompted again after responding.

Written by the indexing model from the issue text.