[bug] Avoid redundant leaflet JS in netjsongraph.js bundle, reuse django-leaflet

[nemesifier]

Depends on and relates to openwisp/netjsongraph.js#425.

It would be great to change the dashboard map so that we load the leaflet JS file shipped in django-leaflet and avoid bundling it with netjsongraph.js.

Current situation:

• dashboard: loads netjsongraph.js which bundles echarts and leaflet (which is huge)
• device/location detail pages: load leaflet from django-leaflet

Ideal situation:

• dashboard: loads leaflet from django-leaflet, netjsongraph.js with echarts bundled
• device/location detail pages: load leaflet from django-leaflet (same file as dashboard, leveraging browser cache)

Follow up:

We can then move netjsongraph.js+echarts to openwisp-utils (openwisp/openwisp-utils#517) and load it from there both here in openwisp-monitoring (#710) and in openwisp-network-topology (openwisp/openwisp-network-topology#272).