MM-45956: Optimize FileInfo stats query (#22603)

* MM-45956: Optimize FileInfo stats query

We Denormalize Post.ChannelId on FileInfo.ChannelId

```release-note
The file info stats query is now optimized by denormalizing the channelID column into the table itself. This will speed up the query to get the file count for a channel on clicking the RHS.

Migration times:
On a MySQL 8.0.31 DB with
1405 rows in FileInfo and 11M posts, it took around 0.3s

On a Postgres 12.14 DB with
1731 rows in FileInfo and 11M posts, it took around 0.27s
```

https://mattermost.atlassian.net/browse/MM-45956

server/channels/db/migrations/mysql/000106_fileinfo_channelid.down.sql

@@ -0,0 +1,29 @@
+SET @preparedStatement = (SELECT IF(
+    EXISTS(
+        SELECT 1 FROM INFORMATION_SCHEMA.STATISTICS
+        WHERE table_name = 'FileInfo'
+        AND table_schema = DATABASE()
+        AND index_name = 'idx_fileinfo_channel_id_create_at'
+    ) > 0,
+    'DROP INDEX idx_fileinfo_channel_id_create_at ON FileInfo;',
+    'SELECT 1'
+));
+
+PREPARE removeIndexIfExists FROM @preparedStatement;
+EXECUTE removeIndexIfExists;
+DEALLOCATE PREPARE removeIndexIfExists;
+
+SET @preparedStatement = (SELECT IF(
+    EXISTS(
+        SELECT 1 FROM INFORMATION_SCHEMA.STATISTICS
+        WHERE table_name = 'FileInfo'
+        AND table_schema = DATABASE()
+        AND column_name = 'ChannelId'
+    ) > 0,
+    'ALTER TABLE FileInfo DROP COLUMN ChannelId;',
+    'SELECT 1;'
+));
+
+PREPARE removeColumnIfExists FROM @preparedStatement;
+EXECUTE removeColumnIfExists;
+DEALLOCATE PREPARE removeColumnIfExists;