File modified fix for Sprint release 2021-03-29

[bjester]

Summary

We had to halt the original release because of an issue with a file migration adding a modified datetime field.

Description of the change(s) you made

See #3074

Manual verification steps performed

1. Ran migrations
2. Checked the database

kolibri-studio=# select count(*) from contentcuration_file where modified is null;
 count 
-------
  3235
(1 row)

kolibri-studio=# select count(*) from contentcuration_file where modified is not null;
 count 
-------
     0
(1 row)