Add migration to change slug for content-design manual

This script depends on the old content design manual having been removed
first.

Once it has been and the slug is available, the migration can be run to
rename the content-design-manual's slug.
Remove AAIB and DFID importers and tests

These imports have been completed and so the code and tests are no longer
required.
Merge pull request #268 from alphagov/remove-aaib-dfid-importers

Remove AAIB and DFID importers and tests
Merge pull request #267 from alphagov/rename-content-design-manual

Add script to rename content design manual
Remove section from the style guide manual

A section has inadvertently been added to the style guide manual which
needs removing. We don't have the facility to unpublish manuals yet, nor
remove them from the (recently added) content store, so we're going to:

1. Remove the manual section from the manual in specialist-publisher
2. Republish the manual manually (heh).

This will make the manual section disappear from the table of contents and
specialist publisher, but it will still show up in site search.
Merge pull request #270 from alphagov/remove-published-style-guide

Remove section from the style guide manual