Keep contributors funded

[karl-dz]

Since these keys have already been allowed into the DoubleZero Ledger network when its Contributor account is created, we should keep these keys funded with enough to satisfy rent for devices, links, etc.

We can use an arbitrary amount of 0.25 credits, which should be plenty.

We may want to calculate how many devices, links, etc a contributor may need to add in the period the funder makes its rounds to transfer credits