Fixup require's of safe_html

The Dir.glob in require_all.rb doesn't load files in a predictable
order, and in some cases was throwing const_missing exceptions because
safe_html wasn't being required everywhere it was being used.
Add indexable_content to TravelAdviceEdition
Merge pull request #47 from alphagov/travel_advice_indexable_content

Add indexable_content to TravelAdviceEdition