Update pricing page with Enterprise Starter volume discounts

Created by: dadlerj

Relies on https://github.com/sourcegraph/sourcegraph/pull/1159

@francisschmaltz I ended up removing the subheaders from each tier — I felt like they were too easy to ignore, and the $10 to start for Starter wasn't getting the visibility it deserved.

Instead, I added it and the new volume discounts bullet to the "features" of the Starter tier.