Remove Singleton & Embedded in favor of Fullscreen with plugin

Alternative to https://git.happy-dev.fr/startinblox/components/sib-chat/issues/127

Implication;

• Remove Singleton & Embedded mode to allow converse to keep connection to already opened rooms (Standard behavior)
• Add fullscreen mode, with a custom plugin to disable the controlbox that contains Contact and Chatroom views UI.

Pros (vs. headless):

• Cost much less time

Cons (vs. headless):

• Less integration than headless. Every custom things that we want must go on custom plugins.

Other pros (vs. current system, also valid for headless):

• Better perfs than current system (No need to leave/join rooms on returning)