Add EMS_ENABLED env variable

[josellausas]

About

This PR adds a environment variable boolean for enabling/disabling EMS Heatmaps.

Relevant Issues

• Closes Add switch for enabling/disabling EMS heatmaps #510

Features

Provides environment variable EMS_ENABLED or change variable default inside settings/base.py
When True, should try to load EMS Heatmap from URL

When False, it should skip all EMS Heatmap

Testing Instructions

1. Go to settings/base.py and change EMS_ENABLED to True.
2. Check that EMS Heatmaps are being shown : http://192.168.33.15:8000/departments/88369/mehlville-fire-protection-district
   (Not all departments have EMS data)
3. Go to settings/base.py and change EMS_ENABLED to False
4. Go to http://192.168.33.15:8000/departments/88369/mehlville-fire-protection-district and check that EMS Heatmap option is not shown.