Add a data migration to rename a gambling duties document collection

- There was a ticket in the second-line backlog about renaming this
  document collection because it referenced "forms", which the page no
  longer housed. Now it only references "notices". It was decided that
  this data migration approach was the most sensible, following on from
  #1861.
- In this data migration, I've also added a line to remove the old
  collection from the search index, so the same titled document won't
  appear twice - thanks Tekin.
Fix data migration
Add content_id field to documents.

Migration to create the field. The content_ids will be populated
separately in a data migration.
Data migration to ensure documents have a content_id

it's better to do this in a data migration so that we are sure that the
application server code has reloaded when this is run.

If this was done in a database migration then there would be a race
condition where:

* all documents had had a content_id set
* application code had not reloaded
* user creates a new draft document which does not get the content_id
  set

We then end up with a few documents without content_ids.
Ensure that content_id is set on new documents

The Edition::Identifiable module handles the integration between
edition and document including the initialization of Documents.
Exported json for an edition should include content_id