Improve source tags and source tracking

As more and more traffic comes from sources that don't expose a real referrer such as X, YouTube, Discord, etc. it'd be great to have a way to push people towards adding a referrer tag such as `?source=foo_discord`.

The issue does not name any files, tests, or entry points. Start by locating the existing source/referrer tracking flow and how traffic from X, YouTube, and Discord is recorded. The intended behavior and completion criteria still need to be defined, including how a `?source=foo_discord` tag should be promoted and stored.

Written by the indexing model from the issue text.