[FIX] Allow inviting other agents to the same LiveChat room

[renatobecker-zz]

@RocketChat/core

Closes #9822
Closes #8234
Closes #8235

This fix allows to invite other agents to join the same current chat.
Now, we are using the _id property as a route parameter, instead of using the code property.
So the room link can be shared with other agents.

Additional information:
When a new agent accepts the invitation to participate in a LiveChat room, the visitor will no longer be notified.

[sampaiodiego]

I was too deep testing this that I sent a fix for:

• using a livechat agent without admin privilegies I was unable to /invite another user. fixed by 70f8833

but the following are still changes I would like to be made:

• link to previous chats are not working (still using code)
• there are references to the deprecated code field across the code base.