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/post.go

@@ -416,7 +416,7 @@ func (a *App) addPostPreviewProp(post *model.Post) (*model.Post, error) {
 func (a *App) attachFilesToPost(post *model.Post) *model.AppError {
 	var attachedIds []string
 	for _, fileID := range post.FileIds {
-		err := a.Srv().Store().FileInfo().AttachToPost(fileID, post.Id, post.UserId)
+		err := a.Srv().Store().FileInfo().AttachToPost(fileID, post.Id, post.ChannelId, post.UserId)
 		if err != nil {
 			mlog.Warn("Failed to attach file to post", mlog.String("file_id", fileID), mlog.String("post_id", post.Id), mlog.Err(err))
 			continue