feat: Implement a notification system for the whole application

[MartinCupela]

Motivation

Provide a notification system that would serve as an independent module / service, exposing API for addition and removal of notification messages.

There is a component that renders message notifications called MessageListNotifications that is tightly coupled to the logic of message list (props hasNewMessages, isNotAtLatestMessageSet, ...)

Current state

1. context provider of notification API - ChannelStateProvider
2. context consumer hook for accessing the API - useChannelStateContext
3. notification rendering component - MessageListNotifications

Acceptance Criteria

Introduction of:

1. context provider of notification API
2. context consumer hook for accessing the API
3. notification rendering component

Some requirements:

1. The notifications should be rendered at the top of the tree, not deeply as in MessageList
2. The display duration of each message notifications should be configurable:
   1. be displayed until removed programmatically
   2. be displayed for a x configurable timeout long period
   3. be displayed until removed manually by clicking a close button
3. Integrators should be able to provide own rendering component => control the visual part of the notifications
4. CSS with configurable theme variables should be included