Send mail to issue/pr assignee/reviewer also when OnMention is set (#18707)

I want to address #17892, where emails notifications are not sent to assignees (issue and PR) and reviewers (PR) when they have the email setting Only email on mention enabled.

From the user experience perspective, when a user gets a issue/PR assigned or a PR review request, he/she would expect to be implicitly mentioned since the assignment or request is personal and targeting a single person only. Thus I see #17892 as a bug. Could we therefore mark this ticket as such?

The changed code just explicitly checks for the EmailNotificationsOnMention setting beside the existing EmailNotificationsEnabled check. Too rude?

@lunny mentioned a mock mail server for tests, is there something ready. How could I make use of it?

#12774 (comment)

Fix #17892

modules/notification/mail/mail.go

@@ -124,7 +124,7 @@ func (m *mailNotifier) NotifyPullRequestCodeComment(pr *models.PullRequest, comm
 
 func (m *mailNotifier) NotifyIssueChangeAssignee(doer *user_model.User, issue *models.Issue, assignee *user_model.User, removed bool, comment *models.Comment) {
 	// mail only sent to added assignees and not self-assignee
-	if !removed && doer.ID != assignee.ID && assignee.EmailNotifications() == user_model.EmailNotificationsEnabled {
+	if !removed && doer.ID != assignee.ID && (assignee.EmailNotifications() == user_model.EmailNotificationsEnabled || assignee.EmailNotifications() == user_model.EmailNotificationsOnMention) {
 		ct := fmt.Sprintf("Assigned #%d.", issue.Index)
 		if err := mailer.SendIssueAssignedMail(issue, doer, ct, comment, []*user_model.User{assignee}); err != nil {
 			log.Error("Error in SendIssueAssignedMail for issue[%d] to assignee[%d]: %v", issue.ID, assignee.ID, err)
@@ -133,7 +133,7 @@ func (m *mailNotifier) NotifyIssueChangeAssignee(doer *user_model.User, issue *m
 }
 
 func (m *mailNotifier) NotifyPullReviewRequest(doer *user_model.User, issue *models.Issue, reviewer *user_model.User, isRequest bool, comment *models.Comment) {
-	if isRequest && doer.ID != reviewer.ID && reviewer.EmailNotifications() == user_model.EmailNotificationsEnabled {
+	if isRequest && doer.ID != reviewer.ID && (reviewer.EmailNotifications() == user_model.EmailNotificationsEnabled || reviewer.EmailNotifications() == user_model.EmailNotificationsOnMention) {
 		ct := fmt.Sprintf("Requested to review %s.", issue.HTMLURL())
 		if err := mailer.SendIssueAssignedMail(issue, doer, ct, comment, []*user_model.User{reviewer}); err != nil {
 			log.Error("Error in SendIssueAssignedMail for issue[%d] to reviewer[%d]: %v", issue.ID, reviewer.ID, err)