better separation of off canvas menu_fixes T1122

Summary: when the off canvas menu is activated the body dims to black while the top bar remains visible

Test Plan:
activate the off canvas menu
confirm background dim to black

Reviewers: rjmackay, shadowhand

Reviewed By: shadowhand

Subscribers: brandonrosage, spaudanjo, sebm90, digitalafrican, srutto, dkobia

Differential Revision: https://phabricator.ushahidi.com/D516