MM-30970 Add Basic unreadMentions support for collapsed threads (#16407)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

model/thread.go

@@ -67,11 +67,12 @@ func (o *Thread) Etag() string {
 }
 
 type ThreadMembership struct {
-	PostId      string `json:"post_id"`
-	UserId      string `json:"user_id"`
-	Following   bool   `json:"following"`
-	LastViewed  int64  `json:"last_view_at"`
-	LastUpdated int64  `json:"last_update_at"`
+	PostId         string `json:"post_id"`
+	UserId         string `json:"user_id"`
+	Following      bool   `json:"following"`
+	LastViewed     int64  `json:"last_view_at"`
+	LastUpdated    int64  `json:"last_update_at"`
+	UnreadMentions int64  `json:"unread_mentions"`
 }
 
 func (o *ThreadMembership) ToJson() string {