dataprovider: add naming rules

naming rules allow to support case insensitive usernames, trim trailing
and leading white spaces, and accept any valid UTF-8 characters in
usernames.

If you were enabling `skip_natural_keys_validation` now you need to
set `naming_rules` to `1`

Fixes #687

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

httpd/api_admin.go

@@ -103,6 +103,7 @@ func updateAdmin(w http.ResponseWriter, r *http.Request) {
 	}
 
 	adminID := admin.ID
+	username = admin.Username
 	totpConfig := admin.Filters.TOTPConfig
 	recoveryCodes := admin.Filters.RecoveryCodes
 	admin.Filters.TOTPConfig = dataprovider.AdminTOTPConfig{}