z-index bug fix

Summary:
* a quick attempt to fix z-index issues

Fixes T1121

Test Plan:
* click the following to check for proper z-index:
  * top bar user info
  * global filter
  * leaflet map
  * off canvas drawer
  * all dropdown buttons

Reviewers: rjmackay, shadowhand

Reviewed By: shadowhand

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

Maniphest Tasks: T1121

Differential Revision: https://phabricator.ushahidi.com/D515