MM-28249 Auto follow threads (#15878)

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

model/thread.go

@@ -22,3 +22,16 @@ func (o *Thread) ToJson() string {
 func (o *Thread) Etag() string {
 	return Etag(o.PostId, o.LastReplyAt)
 }
+
+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"`
+}
+
+func (o *ThreadMembership) ToJson() string {
+	b, _ := json.Marshal(o)
+	return string(b)
+}