Add submodule for django-interval-field.
Embed interval app.
Remove old data references on SportDay.
Replace SportSession time field by an interval field.
Migrate times from time_old to time.
Remove time_old from SportSession.
Work nicely with time interval from forms.
Fix new time usage frmo sport_stats.
Update time displays.
Update migrations to use interval on GarminActivity.
Import garmin time as timedeltas.
Remove time_old references.
Use Garmin activity name when attaching or updating a session, closes #81.
Support Ultra times (> 24h), closes #78.