Change `last_demetra_datestring` column from Varchar to Date

[jonpage]

Also change the column name to last_demetra_date

1. Add a column called last_demetra_date.
2. Parse the string values in last_demetra_datestring and send the result to the new column.
3. Make the related changes in the code (i.e., reference last_demetra_date instead of last_demetra_datestring).
4. Wait a day to see if there are any new related errors in the overnight build.
5. Remove the last_demetra_datestring column.