Bump govuk_content_models to 16.0.0

This includes the change to add a workflow for tags, and to make all
new tags draft by default.
Mark draft tags in the tags list

When viewing the list of tags, mark tags in the 'draft' state with an
appropriate label.
Include the state of a tag in the edit form

When editing a tag, include the state of the tag at the top of the form,
similar to the artefact edit form.

If the tag is a draft, don't link through to GOV.UK, as there won't be
any content there yet.
Only show "take care" notice when editing live tags
Allow a tag to be published

This adds a new 'publish' action to TagsController, and a primitive
'Publish' button to the edit form.

A new Cucumber scenario has been added for tag publishing.
Ensure stub tags in tests are created as live

This changes many of the tests in the app to create tags as 'live' by
default, maintaining the intended behaviour of the tests at the time of
them being written.

This excludes the two importers, as they require more changes. They will
be updated in a later commit.
Move publish tag button to new right-hand column

This splits the form into a layout with better-defined columns, and
moves the “Publish” button to the right-hand column.

We decided to move the button so that it appears conceptually separate
from the main form so that it doesn’t imply "Save and publish" behaviour.
Keep the state of the artefact up-to-date with the tag

This changes the SpecialistSectorTagForm and
UpdateSpecialistSectorTagArtefactObserver so that we keep the artefact
associated with the tag in sync with its state.
Allow draft sectors to be tagged to artefacts

This allows draft specialist sectors to be displayed in the
artefact edit form, so that they can be tagged to an artefact.

To make draft tags appear in the artefact edit form, we have to pass
an argument of `true` to the tag methods on `Artefact`.

To make this play nice with Rails' form helpers and conventions, I've
wrapped this behaviour in a decorator around `Artefact`.
Allow draft sections to be tagged to artefacts

This adds significantly more test coverage around section tagging than
previously existed, and then expands the test coverage to cover tagging
draft sections to an artefact.

To make draft sections appear in the sections select boxes,
ArtefactForm now contains methods to return draft sections for an
artefact.
Fix the MainstreamOrganisationTagImporter

This updates the `MainstreamOrganisationTagImporter` and its tests to
use and create live organisation tags, and to include draft tags when
searching for tags that already exist.
Update OrganisationImporter to create live tags

This changes the OrganisationImporter class so that new organisation
tags are published after being created, and any organisation tags that
already exist in draft are published too.