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/platform/services/slackimport/slackimport.go

@@ -680,7 +680,7 @@ func (si *SlackImporter) oldImportPost(post *model.Post) string {
 				firstPostId = post.Id
 			}
 			for _, fileId := range post.FileIds {
-				if err := si.store.FileInfo().AttachToPost(fileId, post.Id, post.UserId); err != nil {
+				if err := si.store.FileInfo().AttachToPost(fileId, post.Id, post.ChannelId, post.UserId); err != nil {
 					mlog.Error(
 						"Error attaching files to post.",
 						mlog.String("post_id", post.Id),