MM-31182: Reuse the created bot user when creating a DM channel (#16894)

* MM-31182: Reuse the created bot user when creating a DM channel

When creating a bot, we also create a DM channel between the owner
and the bot. So we would hit a replica lag issue when trying to get the
user from the replica instance immediately after creating it.

This falls into the category of read-after-write within a single request.
We fix this by passing the already created bot user during channel creation
itself, thereby getting correctness and saving a DB query.

While here, we also make some other cosmetic improvements:

- Change userId to userID
- Rename nErr to err

https://mattermost.atlassian.net/browse/MM-31182

```release-notes
Fix a bug where creation of a bot would fail due to replica lag.
```

* use correct appError name

app/bot.go

@@ -62,7 +62,11 @@ func (a *App) CreateBot(bot *model.Bot) (*model.Bot, *model.AppError) {
 	} else if ownerUser != nil {
 		// Send a message to the bot's creator to inform them that the bot needs to be added
 		// to a team and channel after it's created
-		channel, err := a.GetOrCreateDirectChannel(savedBot.UserId, bot.OwnerId)
+		botOwner, err := a.GetUser(bot.OwnerId)
+		if err != nil {
+			return nil, err
+		}
+		channel, err := a.getOrCreateDirectChannelWithUser(user, botOwner)
 		if err != nil {
 			return nil, err
 		}