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/app/import_functions.go

@@ -1638,7 +1638,7 @@ func (a *App) uploadAttachments(c request.CTX, attachments *[]imports.Attachment
 
 func (a *App) updateFileInfoWithPostId(post *model.Post) {
 	for _, fileID := range post.FileIds {
-		if err := a.Srv().Store().FileInfo().AttachToPost(fileID, post.Id, post.UserId); err != nil {
+		if err := a.Srv().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), mlog.Any("post_file_ids", post.FileIds), mlog.Err(err))
 		}
 	}