New back-end/update app for IIP and USEP

[atbradley]

@birkin suggested that since IIP and USEP have similar update workflows, they could benefit from a single backend tool to update them. This would:

1. Receive GitHub webhook requests from the iip-texts and usep-data repositories;
2. Identify new/updated/deleted inscriptions;
3. Generate the proper Solr updates using XSL templates; and
4. Post the updates to Solr.

Having a single app to handle this workflow for both projects should simplify administration and updates and make it easier to roll out new sites with a similar update workflow.