MM-46410: adds urgency on mention counts (#20999)

* MM-46410: adds urgency on mention counts

We have introduced priority for posts in
https://github.com/mattermost/mattermost-webapp/pull/10951.
We do need to color the mention badges in the webapp with a prominent
color when a mention is posted in an urgent message.
A thread has urgent mentions if the root post is marked as urgent, and
the replies contain mentions to the user viewing the thread.

This PR adds a column, urgentmentioncount, in channelmembers.
Furthermore when asking for team/thread mention counts, we also return
urgent mention counts for the user.

Adds a new table to hold posts priorities
Refactors priority out of the props and into the new table

We are nilifying Metadata when post.ForPlugin(), which didn't save Priority
for a post when Boards was enabled.
This commit copies metadata again to the post, so metadata are
reinstated.

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Vishal Choudhary <vish9812@gmail.com>

model/thread.go

@@ -41,14 +41,16 @@ type ThreadResponse struct {
 	Post           *Post   `json:"post"`
 	UnreadReplies  int64   `json:"unread_replies"`
 	UnreadMentions int64   `json:"unread_mentions"`
+	IsUrgent       bool    `json:"is_urgent"`
 	DeleteAt       int64   `json:"delete_at"`
 }
 
 type Threads struct {
-	Total               int64             `json:"total"`
-	TotalUnreadThreads  int64             `json:"total_unread_threads"`
-	TotalUnreadMentions int64             `json:"total_unread_mentions"`
-	Threads             []*ThreadResponse `json:"threads"`
+	Total                     int64             `json:"total"`
+	TotalUnreadThreads        int64             `json:"total_unread_threads"`
+	TotalUnreadMentions       int64             `json:"total_unread_mentions"`
+	TotalUnreadUrgentMentions int64             `json:"total_unread_urgent_mentions"`
+	Threads                   []*ThreadResponse `json:"threads"`
 }
 
 type GetUserThreadsOpts struct {
@@ -81,6 +83,9 @@ type GetUserThreadsOpts struct {
 
 	// TeamOnly will only fetch threads and unreads for the specified team and excludes DMs/GMs
 	TeamOnly bool
+
+	// IncludeIsUrgent will return IsUrgent field as well to assert is the thread is urgent or not
+	IncludeIsUrgent bool
 }
 
 func (o *Thread) Etag() string {