Move "deleted" submissions to separate table

[efischer19]

Consider this option 4 in the "how should we soft delete Submissions" balloting.

Testing on my devstack now.

[efischer19]

That's not right, delete() will cascade and get this too.

Hang on...

[efischer19]

Aha! Finally got it work. I can confirm that this behaves as we want in ORA (submission becomes inaccessible both to staff tools and grading pools), and here's what the data looks like on the backend:

mysql> select * from submissions_submission;
+----+--------------------------------------+----------------+----------------------------+----------------------------+---------------------------------------------------------------------------+-----------------+
| id | uuid                                 | attempt_number | submitted_at               | created_at                 | raw_answer                                                                | student_item_id |
+----+--------------------------------------+----------------+----------------------------+----------------------------+---------------------------------------------------------------------------+-----------------+
|  2 | 9e52e9b0-cf4a-11e5-9d07-080027880ca6 |              1 | 2016-02-09 16:31:49.542878 | 2016-02-09 16:31:49.542890 | {"parts":[{"text":"staff response 2 no secrets here"}]}                   |               1 |
|  3 | c2e07982-cf4a-11e5-b5e4-080027880ca6 |              1 | 2016-02-09 16:32:50.868396 | 2016-02-09 16:32:50.868411 | {"parts":[{"text":"honor response I sure hope I don't see any secrets"}]} |               2 |
+----+--------------------------------------+----------------+----------------------------+----------------------------+---------------------------------------------------------------------------+-----------------+
2 rows in set (0.00 sec)

mysql> select * from submissions_submissiondeleted;
+----+--------------------------------------+----------------+----------------------------+----------------------------+----------------------------------------------------------------------------------------------------+-----------------+
| id | uuid                                 | attempt_number | submitted_at               | created_at                 | raw_answer                                                                                         | student_item_id |
+----+--------------------------------------+----------------+----------------------------+----------------------------+----------------------------------------------------------------------------------------------------+-----------------+
|  1 | 84f21e32-cf4a-11e5-ab5a-080027880ca6 |              1 | 2016-02-09 16:31:06.965412 | 2016-02-09 16:31:06.965426 | {"parts":[{"text":"staff response 1 oh no I made a terrible mistake and revealed a huge secret"}]} |               1 |
+----+--------------------------------------+----------------+----------------------------+----------------------------+----------------------------------------------------------------------------------------------------+-----------------+
1 row in set (0.00 sec)