Add Support for Validating Usernames

[svar2388]

Description

I just spent longer than I would like to admit debugging an issue on headscale where the ultimate problem was that the username I had created was not technically valid. Some of the documentation for headscale is a bit confusing and so I ended up creating a username "lm@" which will never match a policy. If headplane could add some minor validation (such as no trailing @) to ensure users don't make the same mistake that I did, that would be really swell.