Add a fake / manual transaction stream

Sometimes, something goes wrong when a donor pays, and they end up paying us using a different mechanism than they usually do. We would like to keep the bookkeeping straight, and ensure they don't fall off the donor page when this happens.

Rather than mucking with the data from Stripe / Every.org, we should add and hook up a transaction stream for these manually added transactions.

No files, tests, or entry points are named. Start by tracing how Stripe and Every.org transaction data reaches the donor page, then determine where a manually added transaction stream belongs; done means manual payments are represented without changing the external data and the donor remains visible.

Written by the indexing model from the issue text.