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/postgres/000106_fileinfo_channelid.down.sql

@@ -0,0 +1,2 @@
+DROP INDEX IF EXISTS idx_fileinfo_channel_id_create_at;
+ALTER TABLE fileinfo DROP COLUMN IF EXISTS channelid;

server/channels/db/migrations/postgres/000106_fileinfo_channelid.up.sql

@@ -0,0 +1,3 @@
+ALTER TABLE fileinfo ADD COLUMN IF NOT EXISTS channelid varchar(26);
+UPDATE fileinfo SET channelid = posts.channelid FROM posts WHERE fileinfo.channelid IS NULL AND fileinfo.postid = posts.id;
+CREATE INDEX IF NOT EXISTS idx_fileinfo_channel_id_create_at ON fileinfo(channelid, createat);