fixed URI and source handling

this was forgotten in 75d29f5d28dd975c0d138a0e8279dc3e45109194
extracted concept mapper's federation capabilities into subclass

with the regular class, there's no "sources" select box now

this improves encapsulation and reduces the surface area, which should
make it easier to reason about

not sure about the automated fallback though - seems kinda dirty?

despite this having been done in rather naive fashion (due to cognitive
collapse), it seems to Just Work - yay?

/cc @mrreynolds