Allow setting created and modified date via API

I'm in the progress of migrating our current knowledge base to Outline. Since the current one has been used for a long time, I'd ideally like to migrate the created and modified dates to Outline. We currently have a small "header" in each document containing these infos, that I'd would like to get rid of.

It would suffice, if I was able to specify the dates on creation. I think a modification afterwards would probably make less sense.