Add enabling and disabling maintenance mode to sites list

[muratulashozturk]

Is your feature request related to a problem? Please describe.
Sometimes we require to shut off a website for maintenance for a while and while we keep it down, we would like to show some kind of maintenance page for the user who enters the requested website. And this process becomes a headache while using nginx.

Describe the solution you'd like

A crystal clear button for enabling and disabling maintenance mode in Sites List.

Describe alternatives you've considered
For alternatively, I currently use https://github.com/tmiland/Nginx-Maintenance-Mode bash script.

Additional context
Add any other context or screenshots about the feature request here.