MM-34758 Collapsed Reply Threads without mobile support (#17424)

Summary
added support for legacy clients accessing server
added collapsed_threads_supported param to viewChannel API and setPostUnread API

Ticket Link
https://mattermost.atlassian.net/browse/MM-34758

Related Webapp PR
mattermost/mattermost-webapp#7933

model/channel_view.go

@@ -9,8 +9,9 @@ import (
 )
 
 type ChannelView struct {
-	ChannelId     string `json:"channel_id"`
-	PrevChannelId string `json:"prev_channel_id"`
+	ChannelId                 string `json:"channel_id"`
+	PrevChannelId             string `json:"prev_channel_id"`
+	CollapsedThreadsSupported bool   `json:"collapsed_threads_supported"`
 }
 
 func (o *ChannelView) ToJson() string {

model/client4.go

@@ -2915,8 +2915,9 @@ func (c *Client4) PatchPost(postId string, patch *PostPatch) (*Post, *Response)
 }
 
 // SetPostUnread marks channel where post belongs as unread on the time of the provided post.
-func (c *Client4) SetPostUnread(userId string, postId string) *Response {
-	r, err := c.DoApiPost(c.GetUserRoute(userId)+c.GetPostRoute(postId)+"/set_unread", "")
+func (c *Client4) SetPostUnread(userId string, postId string, collapsedThreadsSupported bool) *Response {
+	b, _ := json.Marshal(map[string]bool{"collapsed_threads_supported": collapsedThreadsSupported})
+	r, err := c.DoApiPost(c.GetUserRoute(userId)+c.GetPostRoute(postId)+"/set_unread", string(b))
 	if err != nil {
 		return BuildErrorResponse(r, err)
 	}