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/file.go

@@ -274,6 +274,7 @@ func (a *App) getInfoForFilename(post *model.Post, teamID, channelID, userID, ol
 	info.Id = model.NewId()
 	info.CreatorId = post.UserId
 	info.PostId = post.Id
+	info.ChannelId = post.ChannelId
 	info.CreateAt = post.CreateAt
 	info.UpdateAt = post.UpdateAt
 	info.Path = path
@@ -1214,6 +1215,7 @@ func (a *App) CopyFileInfos(userID string, fileIDs []string) ([]string, *model.A
 		fileInfo.CreateAt = now
 		fileInfo.UpdateAt = now
 		fileInfo.PostId = ""
+		fileInfo.ChannelId = ""
 
 		if _, err := a.Srv().Store().FileInfo().Save(fileInfo); err != nil {
 			var appErr *model.AppError