Add a way to filter users by those who have a certain discount applied

[javawizard]

This is per a request from Jen, though I can't find the source at the moment - we would like to be able to view users who have a particular discount applied at the current moment.

This may be tricker than it sounds, because MemberPress creates oneoff Stripe plans for each user and doesn't appear to expose to Stripe which MemberPress-side discount was applied when the user signed up. We may need to talk to MemberPress's API to do that...

...and, given #8, it's debatable whether we should do that now or whether we should wait a bit to see if we're able to do #8 and import all subscriptions, including MemberPress discount data, into Heimdall and save ourselves some trouble.