CKAN background jobs will throw an exception when publishing DOI

[MarkCalvert]

Description
CKAN background jobs can update datasets (e.g. embargo releases), which trigger the after_dataset_update interface.
Flash messages will fail in background jobs and only work from web requests.

Expected Behaviour
No exceptions thrown when a DOI is published from a CKAN background job.

To Reproduce
Create a CKAN background job to update a dataset from private to public

Error Log
Paste any relevant error logs below:

Working outside of request context

Your Setup

• CKAN version: [e.g. 2.10.3]

Anything Else?
Proposed solution #102