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>

db/migrations/mysql/000097_create_posts_priority.down.sql

@@ -0,0 +1,16 @@
+DROP TABLE IF EXISTS PostsPriority;
+
+SET @preparedStatement = (SELECT IF(
+    (
+        SELECT COUNT(*) FROM INFORMATION_SCHEMA.COLUMNS
+        WHERE table_name = 'ChannelMembers'
+        AND table_schema = DATABASE()
+        AND column_name = 'UrgentMentionCount'
+    ) > 0,
+    'ALTER TABLE ChannelMembers DROP COLUMN UrgentMentionCount;',
+    'SELECT 1'
+));
+
+PREPARE alterIfExists FROM @preparedStatement;
+EXECUTE alterIfExists;
+DEALLOCATE PREPARE alterIfExists;