Show registrations from joinmastodon in signup sources on admin dashboard

[supernovae]

Pitch

In the signup sources on admin dashboard, it would be beneficial to know if a registration came from joinmastodon.

Motivation

We don't have much data to operate on, but it would be nice if a registration action came from joinmastodon that the appid for registration be join mastodon. This will help us understand where new users are coming from.