Refactor item status code and add to item search page

[jim]

• Consolidates most of the status display and search code into a single file and names a concept that was always present: borrow status (which is if an item is available, checked out, etc). There are less nested conditionals in a view helper as a result. It also means that we're more consistent about how we were rendering status values for admins.
• Creates two new helper methods to make working with the statuses in views clearer. We now have item_status_label, member_item_status_label, and borrow_status_label.
• Adds tooltips to the two status labels in admin views to make it easier to understand what they mean.
• Finally, adds these status labels to the new item search view.

[phinze]

Great cleanup! Separating borrow status from item status makes everything a lot clearer.

[phinze]

Tiny thing: this no-wrap doesn't actually attach. The only rule for it is .items-search th.no-wrap in admin.scss, so a td slips right past. Adding td.no-wrap to that selector should do it.