Content should not be scrollable when sidebar is shown on mobile

[viki53]

When the sidebar is visible on mobile, the content should not be accessible, just like a mobile app does.

[viki53]

The fix didn't totally resolve it. Need more investigation (or re-do all the sidebar system from scratch)

[viki53]

Issue seems to be solved on Chrome, but not Firefox (Android).

There is also a tiny issue with the toggle button for the sidebar: it's :hover state stays event with the .blur() method being called. The fix should be easy but seems kind of heavy if it's not needed somewhere else…