Push to rummager

Dependent upon https://github.com/alphagov/rummager/pull/308 being released and migrations run.

In the medium term, the search index will automatically be populated when they
are published to Content Store via the Publishing API. For now, though, we need
to push them directly. The same transformation we perform will need to be
performed in the code which populates the search index, so this code may be
reused.
It would be good to add a 'manual' field to the section documents, to allow searches to be performed which are scoped to the manual.  This would require adding the field to the schema in rummager (I commented on the corresponding https://github.com/alphagov/rummager/pull/308).

The idea is to support searching within a manual using this field.