MM-58255 Ensure remote users do not get valid email addresses (#27421)

* remote users don't get valid email addresses; remote users cannot have access tokens

* block notification emails for remote users

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/platform/services/sharedchannel/sync_recv.go

@@ -267,7 +267,7 @@ func (scs *Service) insertSyncUser(rctx request.CTX, user *model.User, _ *model.
 		}
 
 		user.Username = mungUsername(user.Username, rc.Name, suffix, model.UserNameMaxLength)
-		user.Email = mungEmail(rc.Name, model.UserEmailMaxLength)
+		user.Email = model.NewId()
 
 		if userSaved, err = scs.server.GetStore().User().Save(rctx, user); err != nil {
 			field, ok := isConflictError(err)
@@ -321,7 +321,7 @@ func (scs *Service) updateSyncUser(rctx request.CTX, patch *model.UserPatch, use
 			suffix = strconv.FormatInt(int64(i), 10)
 		}
 		user.Username = mungUsername(user.Username, rc.Name, suffix, model.UserNameMaxLength)
-		user.Email = mungEmail(rc.Name, model.UserEmailMaxLength)
+		user.Email = model.NewId()
 
 		if update, err = scs.server.GetStore().User().Update(rctx, user, false); err != nil {
 			field, ok := isConflictError(err)

server/platform/services/sharedchannel/util.go

@@ -95,15 +95,6 @@ func mungUsername(username string, remotename string, suffix string, maxLen int)
 	return fmt.Sprintf("%s%s%s:%s%s", username, suffix, userEllipses, remotename, remoteEllipses)
 }
 
-// mungEmail creates a unique email address using a UID and remote name.
-func mungEmail(remotename string, maxLen int) string {
-	s := fmt.Sprintf("%s@%s", model.NewId(), remotename)
-	if len(s) > maxLen {
-		s = s[:maxLen]
-	}
-	return s
-}
-
 func isConflictError(err error) (string, bool) {
 	if err == nil {
 		return "", false