Yay, we got the API endpoint for this! The `linkage` scope is the key.
Rather than pulling back the specific fallback behavior we had wrote
for usernames before, which was slightly different and involved
appending `neopass` in there too (e.g. `matchu-neopass-1234`), I
figured let's just use a lot of the same logic, and just use the
preferred name as the base name. (I figure the `neopass` suffix isn't
that useful anyway, `matchu-1234` kinda looks better tbh! And it's all
fallback stuff that I expect serious users to replace, anyway.)
Ahh right, in development `User` and `AuthUser` will have the same ID,
but that got messed up early on for us in production DTI 😅
Here, we switch the form to reference the `User` instead of the
`AuthUser` (to get the ID right), then we also change how we compare
the IDs, because `User#to_param` appends extra text onto the ID after
the number!
