Use Level to drive sponsorship prospectus

Rather than hardcoding all of the details about the sponsorship levels
in the sponsorship prospectus template, the `Level` model can be used to
drive the page. To accomplish this the cost needs to be added to the
model.

Additionally, by adding the desired limit on the number of sponsors at a
level, we can automatically indicate when a level is sold out. Using the
`cached-property` library means the sponsor count doesn't need to be
recalculated each time the `is_sold_out` property is referenced in the
template.
Add Sponsors page

This is a page that will display the sponsors for the event. The
sponsors will be grouped by level and displayed alphabetically.

In addition to adding the page, a new property, `accepted_sponsors`, is
being added to the `Level` model. It servers as a shortcut to the
sponsors confirmed for that level, something needed in multiple places.

Closes #16
Merge pull request #34 from PyGotham/sponsors

Sponsors