Add a setting to change map marker icon/color

Summary: Add a setting to change default map marker icon/color

Test Plan: Go to workspace, select map settings and try changing map marker

Reviewers: shadowhand, rjmackay

Reviewed By: shadowhand, rjmackay

Subscribers: brianherbert, srutto, shadowhand

Maniphest Tasks: T434

Differential Revision: https://phabricator.ushahidi.com/D271