Bug 12911: batch_id for new labels batches can be asssigned to several batches

Bug 12911: batch_id for new labels batches can be asssigned to several batches

Currently batch_id is assigned upon creation of a new batch object.  This patch leaves batch_id as 0 at creation and adds a check when adding items.  If batch is new then batch_id is created then

Test plan:
1 -In one browser window, go to tools->label creator and click the new batch button
2 - Before adding items, open a new browser, and go tools->label creator and click the new batch button
3 - Note that both batches have the same number listed
4 - Add an item to the first batch - you should now see one item in the batch
5 - Add an item to the second batch, you should see two items in the batch
6 - Apply patch and repeat steps 1&2
7 - Note that neither batch lists a batch number
8 - Add an item to the first batch, you should see one item and a batch number
9 - Add an item to the second batch, you should see one item and a new batch number

Patch behaves as expected.
Signed-off-by: Marc Veron <veron@veron.ch>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>