support Liberapay: field, with migration support from LiberapayID:
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.