Allow creating and updating (user|org) names with dots

Created by: tsenart

In #4690, we made it possible to create usernames from SSO signups that include dots. But that would have failed, as I found out working on this, due to a database constraint that needed to be updated.

Additionally, it was still not possible to change or manually create an account with a username that includes a dot in the UI. This PR fixes that too.

For consistency, we allow dots in org names as well.

Fixes #4909, #4920