Improve display of long metadata value lists

For metadata that can have multiple values (eg therapeutic_area for a
drug_safety_update document) place each value into a separate `dd`.
Merge pull request #262 from alphagov/improve-display-of-metadata

Improve display of long metadata value lists
Rescue errors when creating a topic in GovukDelivery

When we notify Govdelivery to send an e-mail on publish, we first ensure
the topic exists in the govuk_delivery API.

However, when doing this, govuk_delivery first checks its local mongo
store to check if that topic already exists in GovDelivery. If not, it
attempts to create it.

On preview, development and staging environments, the mongo store is
nuked periodically to sync data with production. This means that staging
and preview lose knowledge of which topics have already been created in
GovDelivery, meaning govuk_delivery attempts to create topics which
already exists.

govuk_delivery therefore attempts to create this topic with GovDelivery,
and then fails.

We rescue this at this level rather than adjusting the function of
govuk_delivery because we're intending to replace this with
`alphagov/email-alert-api`
Merge pull request #263 from alphagov/rescue-govdelivery-errors

Rescue errors when creating a topic in GovukDelivery
Add `dentistry` as a therapeutic area.

This is to be used wherever Dentistry is chosen as a target audience on
MHRA's website.
Merge pull request #266 from alphagov/add-dentistry-to-dsu

Add `dentistry` as a therapeutic area.
Allow deletion of a draft manual via a bin script

This will enable us to remove unpublished manuals, freeing up their slug
for use by other manuals (perhaps which have been made in the wrong
organisation, for example)
Test what happens when attempting to delete a published manual
Merge pull request #265 from alphagov/delete-a-draft-manual

Allow deletion of a draft manual via a bin script
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
Introduce a repository registry

* Slug uniquness validation removed
* All document formats working
* Manuals in need of fixing
Manuals refactored to use repo registry

* Slug uniqueness enforcement completely removed
* Preview broken
Remove slug uniqueness validation code

Fix the broken preview issue where controllers where relying on a plain hash
of errors rather than a ActiveModel::Vaidations object.
Extract entity registries
Slug clash warnings for manuals and documents
Manuals can have multiple clashing sections

(if you hate yourself)
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.