[Self-hosted] Allow existing instances to migrate to no username tags

https://docs.fluxer.app/operator/sign-in/#instances-that-already-exist Currently, existing instances are locked to email instance and random tags on first start-up after upgrade.

Ideally having the option of switching to a username instance and no tags/discriminators for existing instances where the only choice was email and random tags would be preferred. Personally I'm fine with sticking with a email instance with SMTP but would like the option of switching to no tags as I would assume for most self-hosted instances, with the small number of users they are serving, it makes more sense as the likelihood of multiple users wanting the same username are low.