[#44] Guide steps are indexed and analysed from the guide directly.
[#44] Reduce size of index documents for articles.

1. Call uniq on fields which are likely to contain dupes
2. No need to index keywords since they are derived from the content
[#44] Move text analysis callbacks into RailsNlp included module.
[#44] Move search methods from Article to RailsNlp::QueryExpansion
[#44] Analyse guides when guide steps are created/updated/destroyed.