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>

api4/schema.graphqls

@@ -69,21 +69,22 @@ type Channel {
 }
 
 type ChannelMember {
-	channel          : Channel
-	user             : User
-	roles            : [Role]!
-	lastViewedAt     : Float!
-	msgCount         : Float!
-	mentionCount     : Float!
-	mentionCountRoot : Float!
-	msgCountRoot     : Float!
-	notifyProps      : StringMap!
-	lastUpdateAt     : Float!
-	schemeGuest      : Boolean!
-	schemeUser       : Boolean!
-	schemeAdmin      : Boolean!
-	explicitRoles    : String!
-	cursor: String
+	channel           : Channel
+	user              : User
+	roles             : [Role]!
+	lastViewedAt      : Float!
+	msgCount          : Float!
+	mentionCount      : Float!
+	urgentMentionCount: Float!
+	mentionCountRoot  : Float!
+	msgCountRoot      : Float!
+	notifyProps       : StringMap!
+	lastUpdateAt      : Float!
+	schemeGuest       : Boolean!
+	schemeUser        : Boolean!
+	schemeAdmin       : Boolean!
+	explicitRoles     : String!
+	cursor            : String
 }
 
 # Deliberately omitting password, authData, mfaSecret.