feat: add option to disable user settings for changing username or email

[johannes-kirchner]

In an organization it's useful to not allow users to change their username or email.

[thomiceli]

Would be great and safer to also disable the routes.

https://github.com/thomiceli/opengist/blob/897dc43790e1e02128af223b2846e6fd297a0390/internal/web/handlers/settings/account.go#L18 https://github.com/thomiceli/opengist/blob/897dc43790e1e02128af223b2846e6fd297a0390/internal/web/handlers/settings/account.go#L51

Also if there is no email in the first place should we let the users define one ?