Remove "shifted" class from main when nav-sidebar is disabled.

[Amir Ajorloo]

I faced an issue in disabling the nav-sidebar. When I disable the nav-sidebar using ‍‍‍admin.site.enable_nav_sidebar = False, the shifted class is still remaining, because this class is hard-coded in the main tag. Currently check both of shifted class of the main tag and the existence of the nav-sidebar together to apply the styles, but it can be more simple and prevent issues happening in the third-party modules.

In the below screenshot, the nav-sidebar is enabled and it's open. So the main tag should have the shifted class to move its content to right:

In this screenshot, the nav-sidebar is enabled but it's close and the main tag doesn’t have the shifted class:

Bug
In this screenshot, the nav-sidebar is disabled, but the main tag has shifted class (but it shouldn't):

[Amir Ajorloo]

opened nav side bar

[Amir Ajorloo]

closed nav side bar

[Amir Ajorloo]

disabled nav side bar

[Mariusz Felisiak]

​PR