Bug 13593: 'stock number' should be 'inventory number'

Bug 13593: 'stock number' should be 'inventory number'

To make Koha easier to use, we should use terms consistently.
This patch fixes some occurrences of 'stock number' to be
'inventory number' as this is also the term used in the frameworks.

Item search, accessible via the link from staff's advanced search
1) Do a search for items, but choose CSV as output
2) Verify that the header row says 'inventory number'

Acquisition
3) Set AcqCreateItem to 'order'
4) Create a new order, check the labels on the item table in the order

5) Receive the order, check the labels on the item table on receive

6) Set AcqCreateItem to 'receive'
7) Check the item table on receiving an order

Followed test plan (including item search with JavaScipt disabled). Headers / labels display as expected.

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>