Add clustering to MapView

Summary:
- Add clustering using Leaflet markercluster
- Make map clustering a configurable option via map config group

This isn't a perfect solution.. but its a couple of hours quick work on a low priority task.

Aside: simplifying use of leaflet-locatecontrol by fix Init.js lines

Fixes T137

Test Plan:
- Enable clustering in application/config/map.php
- Reload the homepage, markers should be clustered. Zoom in/out etc to experiment.

Reviewers: shadowhand, eyedol, dkobia

Reviewed By: dkobia

Maniphest Tasks: T137

Differential Revision: https://phabricator.ushahidi.com/D145