Check for malformed markup

[octopusinvitro]

Problem

At the moment we don't have a way to detect malformed markup. See for example: #15433

Proposed Solution

Nokogiri in strict mode is probably all we need. (#15433 (comment))

Changing the subject to Nokogiri::HTML(last_response.body) { |config| config.strict } lets us then run a test later like

it "has no HTML errors" do
   subject.errors.must_be_empty
end

Acceptance Criteria

subject.errors must be empty for all pages.