Add API for adding items to a collection.

 * Add route under admin/ for POSTing data to
 * add background worker to do the job

This is intended only for the batch ingest to use. If there should be a
more logical, coherient API for the repository, the batch ingest should
be changed to use that.

DLTP-327
Unused block argument - `i`. You can omit the argument if you don't care about it.<br>Space missing after comma.