chore(username-validation): remove regex

[michaelkaesdorf]

Right now there's a regex on the username property of the UserDto, which is a problem because:

1. It is not configurable (the current regex is a problem for German names 😢)
2. (and more important) Even if it is changed in the source code, the username will still be checked against the UserOptions.AllowedUserNameCharacters Property, which can be configured via appsettings (Defaults to abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-._@+). So you would need to change the regex on two positions.

With the help of this PR, the AspNetCore.Identity Regex will be used exclusively.

This PR fixes #156