Bug 37944: Log object delete as JSON diff, implement for items

Bug 37944: Log object delete as JSON diff, implement for items

We should store deleted objects as diffs just as we do for modification and item created.
This enhancement will store modification diffs if the action is set to DELETE, and an "original" object is passed in.

Test Plan:
1) Enable CataloguingLog
2) Deletee an item
3) Query the database for the newest action log:
   select * from action_logs order by action_id desc limit 1\G
4) Note the diff column is NULL
5) Apply this patch
6) Restart all the things!
7) Delete another item
8) Query the database again
9) Note the diff has been created!

Signed-off-by: Sukhmandeep Benipal <sukhmandeep.benipal@inLibro.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>