In data deletion process, show user number of data points that will be deleted

[larsmennen]

Current state

When a user wants to delete their data using their token on https://epi-collect.org/delete, at the moment as soon as they click submit all their data will be deleted (after it has been verified to be a correct token in the backend).

Desired state

When a user wants to delete their data:

1. They enter their token
2. They see in the UI that this will delete X locations and Y answers to questions they submitted.
3. They can confirm deletion.

Changes required

• In the backend (api.py), the current delete route will need to be split into two: one route that returns the number of data points and one to confirm deletion.
• In the frontend, when submitting, we first call the first route, then show a modal / other UI component to the user stating how many data points there are and allow them to confirm. Then call the second route and delete the data.