Mandate item-level circulation rules RFC

Mandate item-level circulation rules.

Currently we use the system preference item-level_itypes to indicate whether Koha should use the itemtype stored at the bibliographic level or the item level to control circulation parameters. This is one of the more significant distinctions in general policy between US and French libraries, and was also one of the major limitations of Koha 2.x in the eyes of US libraries.

Koha 3.0 offers the option to define your itemtypes at either the bib-level or item-level. However, this option adds unnecessary complexity and offers an easy spot for bugs to appear. I propose that this option be removed for the 3.2 release, and that steps be taken to make this feature-removal relatively painless and invisible for those libraries that do maintain circulation policy at the bib level.

I propose the following:

1. the item-level_itypes system preference be removed.
2. biblioitems.itemtype be renamed to biblioitems.default_itemtype
3. items.itype be renamed to items.itemtype
4. all circulation parameters will be determined by items.itemtype.
5. AddItem and ModItem will automatically fill items.itemtype from biblioitems.default_itemtype if items.itemtype is not supplied.

This should allow French (and others using bib-level circ rules) libraries to continue with their current workflows, and should essentially be unnoticeable to them, while allowing us to remove the bug-prone distinction of item/bib level itemtypes.

Comments and suggestions welcome.