Fix EZP-20300: Design regression with new admin design for extensions when left menu is hidden

In cases when left panel is hidden:
- Hiding left panels separator.
- Wrapping main content into "maincontent" and "maincontent-design" divs.
- Preventing unwanted CSS changes in the head.
- Removing "maincontent-desing" left margin in the head.

JIRA issue: https://jira.ez.no/browse/EZP-20300