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_test.go

@@ -76,8 +76,11 @@ func TestCreateBot(t *testing.T) {
 		assert.Equal(t, "a bot", bot.Description)
 		assert.Equal(t, th.BasicUser.Id, bot.OwnerId)
 
+		user, err := th.App.GetUser(bot.UserId)
+		require.Nil(t, err)
+
 		// Check that a post was created to add bot to team and channels
-		channel, err := th.App.GetOrCreateDirectChannel(bot.UserId, th.BasicUser.Id)
+		channel, err := th.App.getOrCreateDirectChannelWithUser(user, th.BasicUser)
 		require.Nil(t, err)
 		posts, err := th.App.GetPosts(channel.Id, 0, 1)
 		require.Nil(t, err)