add dashboard for privileged user

[ricardogsilva]

This PR adds a new landing page for privileged users where it is possible to download:

• Track segments, as a zipped shapefile
• Bike observations, as a CSV

Other notable changes:

• refactored populatetracks management command in order to allow easier ingestion of real data
• introduced the exportsegments management command that also allows exporting segments as shapefile

fixes #175