[RDY] Offline payments

Flow: Admin Dashboard -> "Record a payment made offline" -> select user, select payment -> submit

Obvious flaws: 
- No protection against the same payment recorded multiple times
- No scoping of users; all users show up in the drop down all the time
- No scoping of payments; all payments show up in the drop down all the time

So some cleanup is required. The basic feature itself is done, though. Feedback welcome. @ivanvarghese