implement channels for chatting in the community

This MR implements the channel communication system in the website and includes all the necessary components to make this a progressive web app (PWA) with push notifications.

There are several todos open:

• implement tests
• implement e2ee (oriented on https://docs.rocket.chat/guides/security/end-to-end-encryption-algorithms)
• implement emoji reactions to comments
• implement autocompletion for sessions, events and contributions
• implement pinned channels for topics, activities, sessions, events and contributions
• implement lazy loading of threads
• implement assigning names for threads
• implement sorting of chat messages either latest first or oldest first
• make the display of channels nicer
• switch to ckeditor version 5 for comments
• make the channel group handling easier