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

app/post_test.go

@@ -2032,7 +2032,7 @@ func TestViewChannelShouldNotUpdateThreads(t *testing.T) {
 	th.App.ViewChannel(&model.ChannelView{
 		ChannelId:     channel.Id,
 		PrevChannelId: "",
-	}, user2.Id, "")
+	}, user2.Id, "", true)
 
 	m1, e1 := th.App.GetThreadMembershipsForUser(user2.Id, th.BasicTeam.Id)
 	require.NoError(t, e1)
@@ -2072,7 +2072,7 @@ func TestCollapsedThreadFetch(t *testing.T) {
 		thread, nErr := th.App.Srv().Store.Thread().Get(postRoot.Id)
 		require.NoError(t, nErr)
 		require.Len(t, thread.Participants, 1)
-		th.App.MarkChannelAsUnreadFromPost(postRoot.Id, user1.Id)
+		th.App.MarkChannelAsUnreadFromPost(postRoot.Id, user1.Id, true)
 		l, err := th.App.GetPostsForChannelAroundLastUnread(channel.Id, user1.Id, 10, 10, true, true, false)
 		require.Nil(t, err)
 		require.Len(t, l.Order, 1)