DateTime is parsed as local time

[Anders55]

When using DateTime.parse to read positions from JSON the timestamp is read as local time, not in UTC. This can cause errors when calculating effort etc.

The problem is related to this issue:
dart-lang/sdk#37420

Need better general handling to solve this, migt need to be handled in data layer and backend as well.
One way might be to handle timestamp without Z as UTC.
This could be defined in the API.
The app should do the same when data is created offline.