Add right sidebar

[savvy-itch]

This PR addresses the issue #53 I've created a right sidebar for the code editor here users can access current room info, chat with the team and view notifications.

There're 2 tabs: Room Details and Notifications.

1. Room Details contains 2 sections: room info and a chat box for communication. Everything is basically placeholder data atm, but I tried keep it as close to the existing TS types as possible.
2. Notifications tab display all the received notifications. I've updated Notification type by adding a isRead property and changing its name to RoomNotification because Notification is a built-in global object and it caused conflicts when I tried to import the type. Created a store for notifications.

Additional changes:

• added collapse menu button for better UX
• moved color theme button to ActivityBar for better UX
• changed some styles because on my screen (1600x900) there was a horizontal and vertical scroll. They should be gone now.

[savvy-itch]

Added a splitter to make the sidebar width adjustable. Now when in collapsed state the width doesn't change, and when the sidebar isn't collapsed the width is adjustable. It came with a couple of trade-off, unfortunately:

• smooth width transition is now gone since Splitter accepts very limited set of styles;
• there's a "magic", invisible div created by Splitter behind the scenes that breaks the layout in the Notification tab a little bit shifting down the notification section.