Don't disclose emails of all users when sending out emails (#4784)

Backport (#4664)

Author: techknowlogick

models/issue_mail.go

@@ -1,4 +1,5 @@
 // Copyright 2016 The Gogs Authors. All rights reserved.
+// Copyright 2018 The Gitea Authors. All rights reserved.
 // Use of this source code is governed by a MIT-style
 // license that can be found in the LICENSE file.
 
@@ -87,7 +88,9 @@ func mailIssueCommentToParticipants(e Engine, issue *Issue, doer *User, content
 		names = append(names, participants[i].Name)
 	}
 
-	SendIssueCommentMail(issue, doer, content, comment, tos)
+	for _, to := range tos {
+		SendIssueCommentMail(issue, doer, content, comment, []string{to})
+	}
 
 	// Mail mentioned people and exclude watchers.
 	names = append(names, doer.Name)
@@ -99,7 +102,12 @@ func mailIssueCommentToParticipants(e Engine, issue *Issue, doer *User, content
 
 		tos = append(tos, mentions[i])
 	}
-	SendIssueMentionMail(issue, doer, content, comment, getUserEmailsByNames(e, tos))
+
+	emails := getUserEmailsByNames(e, tos)
+
+	for _, to := range emails {
+		SendIssueMentionMail(issue, doer, content, comment, []string{to})
+	}
 
 	return nil
 }