Allow funding of Bountysource account unassociated with a specific bounty/pledge

Allow a user to do a transaction directly into their Bountysource account

See IRC log for example use case:
https://botbot.me/freenode/bountysource/msg/5581739/
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Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/869525-allow-funding-of-bountysource-account-unassociated-with-a-specific-bounty-pledge?utm_campaign=plugin&utm_content=tracker%2F47&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F47&utm_medium=issues&utm_source=github).

Start by reading the issue body and the linked IRC log for the example use case. Define the supported direct-account funding flow and its completion criteria before locating the relevant payment entry points; the payload names no files or tests.

Written by the indexing model from the issue text.