Show the participants and drinks in a tabular fashion.

- Add some ordering to the accounts (sort by event, then by first name, then by last name)
- Clean up the code for getting accounts to display in the template
- Change the view from form to template
- Rework the template
Remove the actual form for buying
Change name of "buy drink" view to "sale"
Add functionality for buying a single drink, show error when there are not enough credits left
Allow buying multiple drinks at once
Split the drink buying code from the view code

- Use a RedirectView class instead of the old function
- Clean the old function up and make it use exceptions
- This should make it easier to implement an AJAX method to buying drinks
Make the add credits view class based