Liberapay was originally included using a numeric ID, since they had not yet
finalized the public URLs. Now it is a username. So this logic prefers
the username in Liberapay: field, and uses the old LiberapayID: as a
fallback. LiberapayID: will not override Liberapay: if it is already set.
This reuses the old database key since it is stored and processed as a
String anyway.
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