Bye-bye grey space

We all want to get rid of grey background/complex layout to have more space for awesome GitLab features.

We need to provide 2 approaches for consuming information - fixed centred layout and liquid layout. There is a mechanizm to be considered (https://gitlab.com/profile/preferences)

Check movs below:

Centred layout Centered_layout.mov

Liquid layout Liquid_layout.mov

I will add screenshots for main cases in the comments below cc @dzaporozhets @JobV @jschatz1