[sponsor efforts] Make sponsorship pricing editable without a deploy

[ericholscher]

[From https://github.com//pull/2804/changes/96d178ef5ab0f3b3e08983c9126f518043d47047#r3884264506 ]

The banner sponsorship prices are hardcoded currently as a test. Longer term these should be changable without a PR -- probably a small model + admin. Filing this so we don't lose it.