Create endpoint to register a PDF / markdown conversion

Once a PDF has been uploaded to S3 we want the client to be able to "register" it as being ready for processing. The processing takes time, so this will be an async process (likely batch). This just means that a request should actually be an entity from a REST point of view,and the client should be able to use the resulting entity to look up the status.

This issue is focused on just the POST part, though -- actually registering the pdf for future processing.