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/post_metadata.go

@@ -22,6 +22,9 @@ type PostMetadata struct {
 
 	// Reactions holds reactions made to the post.
 	Reactions []*Reaction `json:"reactions,omitempty"`
+
+	// Reactions holds reactions made to the post.
+	Priority *PostPriority `json:"priority,omitempty"`
 }
 
 type PostImage struct {
@@ -54,11 +57,23 @@ func (p *PostMetadata) Copy() *PostMetadata {
 	reactionsCopy := make([]*Reaction, len(p.Reactions))
 	copy(reactionsCopy, p.Reactions)
 
+	var postPriorityCopy *PostPriority
+	if p.Priority != nil {
+		postPriorityCopy = &PostPriority{
+			Priority:                p.Priority.Priority,
+			RequestedAck:            p.Priority.RequestedAck,
+			PersistentNotifications: p.Priority.PersistentNotifications,
+			PostId:                  p.Priority.PostId,
+			ChannelId:               p.Priority.ChannelId,
+		}
+	}
+
 	return &PostMetadata{
 		Embeds:    embedsCopy,
 		Emojis:    emojisCopy,
 		Files:     filesCopy,
 		Images:    imagesCopy,
 		Reactions: reactionsCopy,
+		Priority:  postPriorityCopy,
 	}
 }