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.

I had to pin various gem versions because otherwise Bundler decided to upgrade
them, with newer versions had been released which weren't compatible with the
app. It may not have been necessary to pin all of them, but it's asking for
trouble to ignore the problem.
Beta badge on published editions view

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

also, beta label checkbox should be disabled when
edition is in scheduled or published state.

this feedback came up during product review.
Merge pull request #247 from alphagov/restrict_allowed_image_hosts

Restrict allowed image hosts
Merge pull request #248 from alphagov/mark-content-in-beta

Beta badge on published editions view
Use section on artefact, edition section's deleted

www.agileplannerapp.com/boards/173808/cards/5444

in order to reduce coupling between Publisher and
Panopticon, we're removing the editions section
field (govuk_content_models v20.0.0).

hence, we should infer an editions section from
the artefact's section.

also includes a migration to unset section field
on existing Editions.