CVAT new UI: enlarge the working are after hiding the right panel

[aschernov]

My actions before raising this issue

• Read/searched the docs
• Searched past issues

Expected Behaviour

It'll be convenient to automatically enlarge the working area after hiding the right panel.

Current Behaviour

There is a large empty zone in the right after hiding:

Context

As our practice shows, it's better to have an opportunity to maximize annotation area. It helps to annotate objects with good quality.

Your Environment

• Git hash commit (git log -1):
• Docker version docker version (e.g. Docker 17.0.05):
• Are you using Docker Swarm or Kubernetes?
• Operating System and version (e.g. Linux, Windows, MacOS):
• Code example or link to GitHub repo or gist to reproduce problem:
• Other diagnostic information / logs:
  Logs from `cvat` container

Next steps

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