guest users shouldn't be added to default channels

Description:

A new Rocket.Chat user with 'guest' role joins the default channel(s).
Our 'guest' permissions are:

• view-d-room
• view-joined-room
• view-p-room
  The guest can receive messages and invitations, but nothing else. So he is not able to see other users except users in the same channel he is invited to.

Steps to reproduce:

1. Create new user with role 'guest'
2. Login to Rocket.Chat
3. Auto-Join default channel with all members and internal-information for not-guest

Expected behavior:

I would not expect a guest user to join default channels - or have a permission to deny it.

Actual behavior:

A guest user with minimal permission joins default channels.

Server Setup Information:

• Version of Rocket.Chat Server: 0.74.3
• Operating System: Debian 9.8
• Deployment Method: tar
• Number of Running Instances: 3
• DB Replicaset Oplog: yes
• NodeJS Version: 8.11.4
• MongoDB Version: 3.2.11

Additional context

Relevant logs:

The issue names no source files or tests, so start by tracing guest-role handling during login and default-channel auto-join, using the reproduction steps as the first check. Done means guest users no longer join default channels while still retaining access to channels they are explicitly invited to, with regression coverage for the behavior.