Fix double geojson load on post detail page. Refs T448

Summary: Trigger MapView.updateMarkers on model change not sync

Test Plan: Load /posts/1 and check geojson is only loaded once

Reviewers: aMoniker, shadowhand

Reviewed By: shadowhand

Maniphest Tasks: T448

Differential Revision: https://phabricator.ushahidi.com/D382