feat: bouttons avec le slider suivant #79

See merge request !1

It is no longer possible to instantiate an abstract model, so we have to
hack around that.

It uses the API to estimate the ground altitude for each point, and then
sets the remaining altitude to "height above ground".  The total altitude
(ground altitude + height above ground) remains unchanged.

To see what would happen with your data without saving the result, run:

    ./manage.py fix_ground_altitude --dry-run

If you're happy with the result, run it for real:

    ./manage.py fix_ground_altitude

Two altitude providers are now used by default:

- a very accurate provider, but limited to France
- as a fallback, a less accurate provider, but with a global coverage

With lots of reference points, the current sorting by distance is really
not usable.  Sorting by name allows to more easily find a given refpoint
in the list.

While we're at it, display the distance in the dropdown menu so that it's
easier to check if it's the right refpoint.