[Feature request] Disallow @ in username

[t-wissmann]

Is your feature request related to a problem? Please describe.
On lmcs.episciences.org, some users have their e-mail address as their username. In my opinion, this can cause confusion, because users can change the e-mail address but not the username (if I see correctly).

In LMCS, we have one user (uid 1049206) with an invalid e-mail address used both in the e-mail address field and in the username field (I don't know how this came to be). I have already fixed the e-mail address, but I am unable to fix the username. So I now need to tell the user to log in with an invalid e-mail address as the username.

Describe the solution you'd like
If @ is disallowed in usernames (during registration), then the distinction between username and email address is made clear.

Describe alternatives you've considered
Alternatively, one could allow username changes, such that we can fix concrete issues manually. (But I think this is much more implementation effort).