MM-40148: threadsOnly query param for user threads (#19833)

* MM-40148: threadsOnly query param for user threads

Currently we always calculate counts when fetching user threads.
Those counts include total, unread replies, and unread mentions,
and are potentially expensive to calculate.

This commit adds a new query param 'threadsOnly' which won't calculate
any counts and just return threads.

Co-authored-by: koox00 <3829551+koox00@users.noreply.github.com>
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

model/thread.go

@@ -67,6 +67,9 @@ type GetUserThreadsOpts struct {
 	// TotalsOnly will not fetch any threads and just fetch the total counts
 	TotalsOnly bool
 
+	// ThreadsOnly will fetch threads but not calculate totals and will return 0
+	ThreadsOnly bool
+
 	// TeamOnly will only fetch threads and unreads for the specified team and excludes DMs/GMs
 	TeamOnly bool
 }