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/post.go

@@ -141,7 +141,7 @@ func createEphemeralPost(c *Context, w http.ResponseWriter, r *http.Request) {
 
 	w.WriteHeader(http.StatusCreated)
 	rp = model.AddPostActionCookies(rp, c.App.PostActionCookieSecret())
-	rp = c.App.PreparePostForClientWithEmbedsAndImages(c.AppContext, rp, true, false)
+	rp = c.App.PreparePostForClientWithEmbedsAndImages(c.AppContext, rp, true, false, true)
 	rp, err := c.App.SanitizePostMetadataForUser(c.AppContext, rp, c.AppContext.Session().UserId)
 	if err != nil {
 		c.Err = err
@@ -420,7 +420,7 @@ func getPost(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	post = c.App.PreparePostForClientWithEmbedsAndImages(c.AppContext, post, false, false)
+	post = c.App.PreparePostForClientWithEmbedsAndImages(c.AppContext, post, false, false, true)
 	post, err = c.App.SanitizePostMetadataForUser(c.AppContext, post, c.AppContext.Session().UserId)
 	if err != nil {
 		c.Err = err
@@ -479,7 +479,7 @@ func getPostsByIds(c *Context, w http.ResponseWriter, r *http.Request) {
 			}
 		}
 
-		post = c.App.PreparePostForClient(c.AppContext, post, false, false)
+		post = c.App.PreparePostForClient(c.AppContext, post, false, false, true)
 		post.StripActionIntegrations()
 		posts = append(posts, post)
 	}