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 reverse_markdown because Bundler decided to upgrade it to a
version with API changes that broke our usage of it.

I had to pin webmock as Bundler decided to upgrade it to a version that caused
some apparently unrelated failures in LocalInteractionImporterTest.