* Homogeneize the handling of opendata files.
* Use a clear distinction between geometry (the position of the report)
and geo_shape (the shape of the report area).
* Keep track of OpenData sources, fix#51.
* Keep track of the area covered by a given report.
Upvotes now reset the `datetime` field of the report, making it appear
as if it was newly reported. They also extend the lifetime for accidents
and GCUM.
This introduces database migrations as well.
Closes https://framagit.org/phyks/cyclassist/issues/41.
