Rework reservable item listing and reservation building.

[jim]

• Store reference to in-progress reservation in the session
• Make the reservable item pool listing generally browsable (add a link in the nav)
• When a reservation is in progress, show a banner at the top of the page that shows its dates, how many items are in it, and a button to submit the reservation.
• When a member is viewing an item pool page and they have a reservation in progress, show the "add to reservation" button. When it's clicked, add it.
• Replace the "Add Items" button on reservations to a link to the item pools index.