Information source for merges

[tfgg]

It'd be useful to be able to give a source when merging candidates, in order to record the evidence that they're the same person somewhere. E.g. "This photo from 2010 (X) is the same person as this photo form 2015 (Y)"

[andylolz]

Also (useful in a minority of cases) evidence for the directionality of the merge e.g. “This person is definitely standing in constituency A, not constituency B. [Source that ideally explains the confusion]”