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.go

@@ -71,6 +71,10 @@ const (
 	PostPropsGroupHighlightDisabled   = "disable_group_highlight"
 
 	PostPropsPreviewedPost = "previewed_post"
+
+	PostPriorityUrgent               = "urgent"
+	PostPropsRequestedAck            = "requested_ack"
+	PostPropsPersistentNotifications = "persistent_notifications"
 )
 
 const (
@@ -158,6 +162,15 @@ type PostReminder struct {
 	UserId string `json:",omitempty"`
 }
 
+type PostPriority struct {
+	Priority                *string `json:"priority"`
+	RequestedAck            *bool   `json:"requested_ack"`
+	PersistentNotifications *bool   `json:"persistent_notifications"`
+	// These fields are only used internally for interacting with DB.
+	PostId    string `json:",omitempty"`
+	ChannelId string `json:",omitempty"`
+}
+
 type SearchParameter struct {
 	Terms                  *string `json:"terms"`
 	IsOrSearch             *bool   `json:"is_or_search"`
@@ -306,6 +319,7 @@ type GetPostsOptions struct {
 	FromCreateAt             int64  // CreateAt after which to send the items
 	Direction                string // Only accepts up|down. Indicates the order in which to send the items.
 	IncludeDeleted           bool
+	IncludePostPriority      bool
 }
 
 type PostCountOptions struct {
@@ -770,3 +784,20 @@ func (o *Post) GetPreviewedPostProp() string {
 	}
 	return ""
 }
+
+func (o *Post) GetPriority() *PostPriority {
+	if o.Metadata != nil && o.Metadata.Priority != nil {
+		return o.Metadata.Priority
+	}
+
+	return nil
+}
+
+func (o *Post) IsUrgent() bool {
+	postPriority := o.GetPriority()
+	if postPriority == nil {
+		return false
+	}
+
+	return *postPriority.Priority == PostPriorityUrgent
+}