This addresses a request to make the links optional and instead allow a
text field for the date of diagnostics to indicate that they were
completed.
The field is now a regular text field with no validation, and does not
prescribe any particular format of the input (not even a date). The
field names are left unchanged for backwards compatibility (ie. not
having to change them anywhere else). Practice shows that the URL
feature was not used, so we can change the name in the exported and
aggregated data, for example.
