Include search terms in feed link
Use the features of ransack to filter in the database

Instead of loading all the models into memory. This should make
search more performant now that we have several thousand certificates.
Ability to filter by domain or dataset documentation_url

Use domain=data.gov.uk in query string of index page
Merge pull request #951 from theodi/feature-feed-filtered-by-search

Feed improvements
Also generate certificate if only draft certificates exist
Extract params checks to before_filters

The if statement was getting too unruly to be readable.
Clean up create method

By extracting all negative responses to before filter checks.
Memoizing the creation of campaign - sorry about the :nil_but_true
Pass existing dataset through params hash to method
  (I wish ruby had decorators)
It’s the certificates published state that matters