Index like other manuals

[rboulton]

Index documents using the "manual" formats This depends on alphagov/rummager#399; I'll deploy both that and this change together.

So that we can move to using the unified_search endpoint, we want to index documents from the service manual the same as if they were from one of the standard manuals on GOV.UK (ie, using the recently introduced manual and manual-section formats).

This requires several changes:

• update version of rummageable, since the old one doesn't allow anything except for a very limited set of fields to be sent.
• the documents from the manual need to be indexed with their type set to manual_section.
• manuals are generally associated with an organisation, so associate this manual with the GDS organisation.
• the titles in search of all existing manual_section documents are prefixed with the name of the manual. This makes them make more sense when they come up in general site search, but is undesirable when the search is restricted to a single manual. Therefore, we also strip this prefix off again when presenting the results in this app.
• for search-within-a-manual to work fully, we need a manual document to describe the manual. This is a good idea anyway, because we're currently using the "recommended-links" app to add a link to search, so that a search for "service-manual" on GOV.UK finds it. This document requires a "description" field, for which I've copied the existing description used for the link in the "recommended-links" app.

This PR shouldn't change the results shown to users at all.

[bradwright]

Small comment about the description used in the general search index, otherwise the code looks good.