Fundamental bug in the offline repository

[tdybda17]

Describe the bug
Models are stored in plain .json format in the offline repository which can produce unexpected behavior when reference models are updated or deleted.

Say an Activity with offline_id = 45 is saved in the offline repository with a pictogram with offline_id = 34:

{
    "id": "1",
    "order": "20",
    "pictogram": {
        "id": "2"
    }
}

If then the pictogram were to be deleted with

PictogramModel.offline().delete(34)

Then calling ActivityModel.offline().get(45) will still return the activity with the deleted picogram.

To Reproduce
Steps to reproduce the behavior:

1. Insert an activity to the repository
2. Delete its pictogram
3. Get the activity again
4. Notice that the pictogram is still there

Expected behavior
Activity should not get the deleted pictogram with it.

Actual behavior
Activity gets the deleted pictogram with it.

[Riasalit]

This issue does not exist anymore as all previous offline functionality was removed by 2020f