Editions can be marked as being in beta

www.agileplannerapp.com/boards/173808/cards/5418

As a GDS content designer
I want to show GOV.UK end users that a piece of
mainstream content is not yet the official source
and is still in beta
So that I can trial content with the public and
gather evidence to make it even better
Merge pull request #246 from alphagov/mark-content-in-beta

Editions can be marked as being in beta
Restrict allowed image hosts

This prevents images not hosted by us from being included in content on GOV.UK
by content editors or an attacker who has gained control of an editor's
account. Such images could be altered to something malicious (or a trollface)
at the whim of the website owner or an attacker who gains control of that
domain.

This is provided by version 19.0.0 of govuk_content_models.