Sanitise admission_date based on consultation suggestion

[rithviknishad]

Describe the bug
Currently, we are capturing admission_date only for "domiciliary care" and "admission" decision after consultations. However, since we are allowed to edit the "Decision after Consultation" the previously entered admission_date is not removed.

To Reproduce
Steps to reproduce the behavior:

1. Go to a "admission" consultation
2. Edit the decision after consultation to anything other than admission / domicilary care.
3. Submit and see the response of request, admission_date is not cleared.

Tasks

• Set admission_date to null for when the field is hidden.
• Custom migration to sanitise existing records in the backend.