Add focus/full screen mode

[dwivedikunal100]

Current behavior

There is not focus mode, the side bar is persistent

Expected behavior

A button to enable focus mode

Steps to reproduce

Present in old boostnote

Environment

Desktop/Web

• Boost Note.next version: 0.9.0
• OS version and name: Mac Os Catalina

[Flexo013]

#448 will be done before this feature. Afterwards we will look into this feature.