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/migrations.list

@@ -210,6 +210,8 @@ channels/db/migrations/mysql/000104_upgrade_notifyadmin.down.sql
 channels/db/migrations/mysql/000104_upgrade_notifyadmin.up.sql
 channels/db/migrations/mysql/000105_remove_tokens.down.sql
 channels/db/migrations/mysql/000105_remove_tokens.up.sql
+channels/db/migrations/mysql/000106_fileinfo_channelid.down.sql
+channels/db/migrations/mysql/000106_fileinfo_channelid.up.sql
 channels/db/migrations/postgres/000001_create_teams.down.sql
 channels/db/migrations/postgres/000001_create_teams.up.sql
 channels/db/migrations/postgres/000002_create_team_members.down.sql
@@ -420,3 +422,5 @@ channels/db/migrations/postgres/000104_upgrade_notifyadmin.down.sql
 channels/db/migrations/postgres/000104_upgrade_notifyadmin.up.sql
 channels/db/migrations/postgres/000105_remove_tokens.down.sql
 channels/db/migrations/postgres/000105_remove_tokens.up.sql
+channels/db/migrations/postgres/000106_fileinfo_channelid.down.sql
+channels/db/migrations/postgres/000106_fileinfo_channelid.up.sql