Improve "Shopping Cart" on Opportunities screen

The purpose of this ticket is to improve usability of the shopping cart and ensure that volunteers do not abandon the sign-up process before completion. The concern is that volunteers may believe they've completed the sign up after adding the volunteer opportunity to their "cart" (i.e., when they've checked the box for an opportunity of interest).

To address this, we will:

• Redesign the cart element (i.e., the box labeled "Your Selected Volunteer Opportunities") so that it follows users down the page should they scroll beyond where it normally appears.
• Redesign the cart element to show how many items are in the cart and to provide some instruction. For example, "You have selected 2 volunteer opportunities. Add more by clicking the checkbox for the opportunity that interests you, or click 'Sign Up!' to complete your registration." The contents of the cart (i.e., the display as it exists today) will be hidden by default; visitors will be able to view/edit these items by clicking a button.
• Add subtle animation (perhaps a short "glow" effect) to both the volunteer opportunity and the cart element when an item enters or leaves the cart. This can be achieved by watching the collection of items in the shopping cart and registering a listener to fire when changes occur.