digmill use api to ensure no duplicate digmill numbers

[annakrohn]

perseids-project/digital_milliet#32

[annakrohn]

See commit d8f8d0d

@balmas, this is a super simple alert, with a bit more time it could be more complex, a dialog box, link to the record, etc. Do you think it is worth it?

[balmas]

I think it's okay as simple for now, although it would be nice to generalize all of this a bit more at some point (i.e. so that it's not necessarily specific to milliet numbers but any identifier which we want to center the publication on) but I think that's something for another round.

But see my question on the code in the commit itself.